I think the paintbrushes would be gone by the afternoon. I really always tell people to try everything at least once, but honestly, I think tom sawyer island was a waste of time. There were so many pitch black dark caves that my DD was terrified of and to be honest, so was I. You can't see where your going and you can easily loose your child in a cave or anywhere on TS island. I didn't like the rickety bridges either and I think thats the only way to get around.
To me, I'd rather be getting on line for attractions and doing something better with my time than spending time going to TS island and hoping to possibly find the paintbrushes.