Obvious warning: this toy contains a tiny gem that can break off and be eaten by a child. Don't give it to a kid that's going to do that. If you aren't sure, don't give it to a kid at all.

The interior diameter is 15mm and fits my daughter's fingers for now (she is almost 5). You could scale this model up for larger fingers.

To print: split the STL into two pieces and print separately. The gem should be printed upright and with a brim.