Expect this to happen also as the kids' hands get bigger and stronger and as motor skills improve.
Regarding pronaton specifically, two things to watch for and to avoid are:
(1) Allowing the throwing arm to "sweep" across in front of them. Sometimes, in an attempt to pronate, their whole throwing arm sweeps across towards the glove side instead of staying on path to the target. That produces poor results.
(2) Late posture shift. Sometimes, in an attempt to pronate more than their flexibility allows, kids shift their posture to the glove side to help the arm come through with pronation. They should only pronate to the degree that's still comfortable and for which posture can be maintained.