Agreed.Teach her how to throw correctly first and the rest (e.g. spin) will take care of itself. See Wasserman's stuff.
In terms of throwing sidearm, at 3B it won't be used as much as MI but there are some balls (in particular bunts) where throwing from a lower armslot will be helpful. Plus she is 9, she may end up in the MI at some point. I would just play around with it now when you playing catch. Be ready to chase the ball at first