Hi Erika,
We have a few articles about using resin as a coating that you should find helpful: https://resinobsession.com/?s=resin+as+a+coating
In general, however, I don’t know that the resin will make the sea urchin unbreakable. It will certainly make it less fragile, but I don’t know that it would survive a fall if dropped, for example.
If I were to attempt this project, I would use the Resin Obsession crystal doming epoxy resin. It is clear, doming epoxy resin and should give a nice coating to your projects. You can find it in our store here: https://shop.resinobsession.com/collections/resin/products/resin-obsession-crystal-doming-epoxy-resin-jewelry-grade