Hello morah,
I think your situation is not so very simple, having children from 5-13 years, only 45 minutes a day and around 20 children at once! My great respect to you!
I´ve experience with classes of 18 children, who not really wanted to do clay projects but had to attend them because of the schools afternoon schedule.
It was really a horrible situation for me, (2010) teaching simple projects nearly prohibited by those children who tend to use clay, paper and water to create throwable little bombs.. ;-)
I claimed at the school to change their system to only maximum 10 kids (13-14years) (1,5 hours) and to give the children a possibility to choose the clay work freely.
Now the situation is much better and I can really give children a chance to learn something and get projects done.
But simple projects are not possible for all children, some tend to destroy their work while doing it (especially boys) and so they have to start again from zero or let it be and do something really simple like a little snake or something else.
I keep projects simple so they have the possibility to learn by doing, plate technics, stamping/rollng on structure etc. some mold technics.
But if they don't listen and don't follow intructions, hey, it's not me who have to do the work again!
Poking in holes in some too thick projects from the bottom or through the eyes is one possibility to help keeping things not blown up in the kiln.
It is rather easy to do this "fixing" during the lessons or after the lesson. Explaining it during the lesson helps to give children a chance to learn and avoid mistakes.
I stress from the first lesson, that they shouldn't work in air bubbles (e.g. by destroying a project, putting it again in a poorly slapped pack and rolling out again).
I give some too massive things back to them after drying without kiln firing (not possible) to get it home if they wanted.
At least, my aim is that by trying easy clay concepts, they get things to bring them home. But I have once a week, half a schoolyear as time for this.
In a summer camp, of course you have different conditions. Making medaillons to get home is a brilliant idea, I think. snakes are always great for boys. And fishes are really a good thing ,easy to do.
Maybe think about acrylic decoration instead of glaze. its fast and children love it (what you see is what you get) . smaller children may do it with water colors.
If they are modelled massive and not bigger than 10-12 cm, its a good idea to open the mouth and poke in a nice hole (together with the child, not after the class). It normally won't blow up.
Nice snails are also easily done projects, as well as mushrooms done from a ball without "scoring"..
birthday party.jpg
This picture is from a birthday party, 10 years old. I did first snails (to coils, one to get the house, the other will be the snails body, just a little "scoring and smoothing" to get them together. Decoration was fun for the kids. ) Poke in one hole from the bottom.
then as second project (1,5 hours) they tried plate technics, and other easy things.
birthday party2.jpg
Here another party with children 12/13 years old.
birthdaysecond.jpg
birthdaysecond2.jpg