26″ for the older, you can get 27.5s that’ll fit but there is an argument the smaller wheel aids skill development. Also keep an eye on crank length. Should be around 145mm for that height, you’ll probably have to swap those though as most kids bikes are fitted with cranks that are too long.
24″ for the younger but be prepared to swap cockpit and cranks lengths to get the fit right.
Both should be good with proper gearing.