If it smells I would dig it in a corner of the garden in compost earth and keep the earth humid to clean it from meat.
Than I would brush it under the tap and put it on a stone in the garden that wind, rain and sun can clean the rest.
That can take half a year without any question or even more.
This epoxy block is a nice idea, but I wouldn't teach a child to use unnecessarily plastics.
I would put it on a bit smooth stuff in a transparent Ferrero sweets plastic box or something like that, close it with an elastic strap, and so the child can carry it carefully to school.
If it breaks, it breaks. Doesn't matter.
But bone doesn't break so fast.
After that opening I would watch out, where I can get skulls of other animals from so that it becomes a collection.
My brother had a whole cupboard full of this stuff and became a farmer after he studied veterinary medicine.
Real professional interests are awaked in this age. Such people become outstanding good in the end.
(But of course, you should do other interesting things with them too.)
