Would it be illegal for girls in bikinis under the age of 18 to wrestle in pudding?

At home in your bathtub no. In public, that would depend on where you live and perform. If some one is charging a fee to show it, that would be illegal.

The more important question is why would you do this. If you answer "for fun", you are lying to yourself and anyone else you tell this. If it was fun, you would be doing it at home in your bathtub. If you can honestly figure out the real "why", you will learn something.