How do I make this rattle in this picture, I tried making it out of rice crispies but can't get it smooth. any ideas?

Depending on how big you want it you could mold some fondant or gumpaste in a 1/2 ball, then glue them together w/royal or choco.
If you don't have a 1/2 ballpan, mold it over an actual ball. It should dry enough overnight unless it is very, very humid where you are.

Maybe use 2 different size styrofoam balls covered in fondant with a dowel inserted into both balls. just an idea hope that it helps