My son's first rifle was a 223 handi with the standard 22" barrel. He used a shooting stick and a shooting rail in the tree stand to steady. Like quick said, no recoil to worry about. He killed his first deer with it and a friend of mines daughter killed her first with it this year. Note that shot placement is everything, especially with a 223, so have her practice shooting it in hunting positions and not just off of a bench if she is going to hunt with it.
PS: You can usually find stand length rifles used easier than youth models and you can just buy a youth stock set for it. Keep the standard LOP set and put them back on when she grows or anytime you want to use it. It takes less than 5 minutes to swap them...