David is right, keep it simple.

Here's Jackie's recipe for paste: http://www.papiermache.co.uk/tutorials/ … ier-mache/

Use the one for the thinner paste. Doubling the amount will make about 2.5 quarts or litres.  If you think that won't be enough, X4 the original recipe.  You'll be in trouble if you don't make enough.  You can dump leftovers (garbage or compost pile).  One cup of flour makes a gallon, so it's cheap enough to make more than you need.  If you don't refrigerate it, it will start to mold.

Where Jackie says to keep stirring, KEEP STIRRING, or you'll get semi-cooked lumps (like in gravy).

All the kids with long hair need to tie it back before beginning.  And watch the boys, esp if they're near a girl with long hair. Trust me on this!

Plan ahead what smaller containers you're going to put the glue in, and how it's going to be applied.  Dip and squeeze, or brush it on?  Don't use any containers that have a smaller base than the top, they tip over too easily.

Newspaper is fine, these aren't going into a museum.  But if you want more rigid results, use the brown or white paper as David suggested.  But these things probably won't last a week, so keep the expense down.

Round things like balloons are easier to work on if you set them in a container, like a larger empty tin can.  Otherwise, they roll around while you're working on them, and the wet part (heaviest) always rolls to the bottom.

There is always a temptation to use large pieces of paper to get faster coverage.  Don't, as the lumps will multiply.

The balloons will dry more evenly if you hang them, so tie the necks with a length of string as soon as you blow them up.  I hope you've got help blowing them up, that's a lot of air.  If you've got some politicians handy, put them to work.

If you can string a clothesline rope across the room, you can hang them from that for drying.

If you have a bunch of old towels, wet them and wring them out for wiping hands, and place them around the room for easy access.  As the kids work, the glue will start to dry on their hands.  The easiest way to do this is to drop them into a washing machine and then spin the excess water out of them, put them in a plastic bag for transport.

If they're going to paint them when they're dry, practically any kind of paint works with flour adhesive:  tempera, acrylic, wall paint, etc.  Just make sure it's water-based, not oil-based.

Watch the boys!

If you volunteered for this emergency, you will probably have learned your lesson by the time you're finished! *big grin*

Sue

Sue said, "As the kids work, the glue will start to dry on their hands.  The easiest way to do this is to drop them into a washing machine" . . .she means the towels!! lol
One further tip.  Get them to use as little paste as absolutely necessary.  The paper only needs the thinnest coating.  They can dip their fingers in water as they work if things are getting too sticky.
About six layers of paper can be really strong.  Twelve will withstand a lot.  Make sure they are totally dry before popping the balloon, or even letting them deflate, otherwise they will pull the whole laminate in with them.
DavidO

Nope, I think you'll be alright.  I like the bit about breakdown - "I'll have another 10 minutes to break down again".  You're probably right ha ha.
Legs etc.  Should be easy.  Can you get hold of egg boxes?  Preferably the paper composite type?  Wait till the basic balloon shape is dry, then cut out each piece of the egg box for legs and snout.  Don't use composite - it will take too long to dry.  If they laminate over the promontories (say four layers), it will hold them firmly in place.  Where you put the money slot, the layering needs to be quite firm (ten layers minimum?) and then sawn through with something like a serrated kitchen knife.
DavidO

A jig saw will be fine, providing you use a fine blade.  Drill a hole first that is large enough to take the blade.  Probably two holes would be a good idea, one for each end then jig the the slot between them.  This is presuming you have made the laminate strong enough in that area as I described above.  Otherwise, yes, it would tear.
DavidO

Given the assessment of scissor skills, you're dealing with 5-year-olds, < or >.   25 in one batch is huge.   Will you have assistance? -you'll need it.

Since you will be storing the work over several weeks, and need to redistribute, you will need to label the work- say, strings tied to the balloon stems with attached tags.   Once the shell is dry, you can use a paper punch to transfer the string to the shell.   If you pre-label the tags, you can remove that time from what you budget in class.   If you do it in-class, it may cost 10% of your class time (and the children's attention).   Of course, if they are able to write their names, make sure it is with a blunt instrument- or you may have to deal with popped balloons.

Drilling and cutting slots for 75 sounds like a nightmare; if the plan is to break the bank to remove the money, you won't have an access hole to support the work from when you drill and cut.   Consider taping a thin piece of foam/sponge to the balloons (broad masking tape sandwiching the foam): they can paper around/over it- then just cut it off with scissors, and you have a slot.

If you end up using paper tubes for the legs, consider pre-marking the cut lines- although, IF they are 5, the cutting task will likely be a frustrating failure; their level of coordination combined with those wretched little scissors and a couple layers of card stock is too formidable.

Make an example of each stage- it will help them, and allow you to see where they are likely to have difficulty.

Anyway- just my 2 lira worth....

Yes, you can thin the paste with water, not too much though.  Aim for thick cream consistency.

Hey guys - Catperson, Mavigogun etc.  How about we all go over and help Megan.  Now THAT would be fun!!
DavidO

Well... I survived! 

lol, we have some very shabbily covered balloons and even more only partially covered, but I made it through the day and only have a slight headache. 

I wanted to thank everyone for their suggestions and advice - I never would have made it through this day without it!  That's one Monday down, only 4 more to go... I think I've earned a glass of wine!

Definitely
DavidO

ugh... about 50% of the balloons deflated!  I had to scrap the piggies in favor of mask making instead.  I put in a rush order this morning for some 2D cardboard mask cut-outs, onto which the kids are going to build up paper mache facial contours.

I've also been playing around with the egg cartons that I had the children collect and bring in this week.  If I soak them in water, they become very squishy and workable. I think they'll be able to use them for 'puffing out' cheekbones etc.

Well, last week was spent preparing for our summer fundraiser so, their art class time was occupied with costuming and finishing the sets.  I wasn't able to work with the kids on their long term paper mache projects but, we'll begin them again this Monday.  Thanks for the tips pinata pimp - much appreciated!

Rubber balloons deflate no matter how you tie them, but tying the neck in a knot makes it last the longest.

I wonder if they would last longer if you sprayed them with shellac or something?  Just an idle thought.

David, a friend of mine filled a latex glove with plaster and set it palm-up in a bowl to give a relatively natural curve to it.  When set, she drilled a hole through the palm and inserted a large nail.  When cured, she painted it flesh-colored and mounted it on a plaque for a friend, as a reminder not to test a nail gun with his hand in front of it... again.

Sue

If you blow balloons up to their full extent and then deflate them a little they also last longer. New balloons are better, if they've been on the shop shelves or in storage for years they're often past their best.

Good luck!