I have no answer for you. But I think a kid's ball looks right at home in the garden as is.

I haven't done one yet but I was thinking about it the other day. Most basketballs can be aired up again. I was going to try one using a spray paint that is glittery and made to use on plastic. Some where I read that when painting plastic you should cover the area with Elmer's glue, let it dry and then paint it. But that was before a paint for plastic was available (or at least before I discovered a paint for plastic). Who knows? It just might work. The second hand stores are getting wise to the project of using bowling balls and in this area are charging $5 and up for a ball. Any way.....have fun!

Fill them with cement or hypertufa (there is a forum here on hypertufa) and make cement garden balls.