The little robot is quite interesting.  Probably more so for a budding teen who can get to grips with it all than a 6 year old and her father who's some sort of engineer already.  But this meets in the middle.

The first thing to sort out was to give it a silly hat.  Something that between a hat and a bowl seemed a good place to start (as a bowl just sitting on it's head with a Nerf ball in it worked well).

Then the arms - it only had 1.  A pair seemed sensible, but modified to allow marbles to skitter about in them.

Now it's more fun to look at and stack stuff on, but the PID could do with a tweak.  Well, it could have done with a tweak before increasing it's moment of inertia.  That's for next time.