Is it okay to wiggle your fingers with a broken wrist. it is in a hard splint, to be castes next week?

Absolutely: In fact, it is encouraged in order to minimize swelling. Gripping a sponge or ball helps this. Also, stretching the fingers is also helpful to minimize stiffness for after the fracture heals.

Yes in fact: It is good for circulation, preventing stiffness and reducing pain. While the first time fingers move it may hurt more often it will feel better once they get going.