wire brush is good for removing rust. though will have a detrimental effect on any surrounding plastic.

as for removing screws, try using a hardened steel (pin, sewing needle or mini screwdriver) to scratch out enough from the screw head to fit in the screwdriver.

in larger applications it helps to hammer on the end of the rusted screw to loosen it, don't know how well this would translate to toys. perhaps just push down hard while trying to unscrew.