I've reused rims, no problem. It's not as easy or perfect as using new ones, but for training wheels, who cares? I like to use parts until they are completely worn out- waste not want not and all that. If they have braking surface left, and aren't wildly bent, go for it. Be prepared for the spoke tension not to be perfectly even, and take the old wheels apart with a spoke key, not a pair of clippers.