Strength will always be an issue because some is good and more is better, and I don't know if there's such a thing as too much. Putting strength aside for a moment, I'm gonna focus on what it takes to do a roundoff and say that it's a skill that relies on correct positions and working on it using the right concept. I'm in the same car as BlairBob with the concept of starting the skill like a front handspring that is allowed to drift into a 1/4 turn that's a consequence of landing on the front foot after the hurdle and kicking the back leg. The front foot acts like an obstacle that forces the opposite side to begin a turn.... kinda like roller skating (or ice) and dragging one skate will force you to focus on keeping strait unless you want to turn. The rest of the motivation for the turn comes from the hip tilt caused by kicking the back leg to the limit just as you'd experience in a split leap.
So tell the kid the last thing you want is for her to try to turn. Her concentration should be on supporting her weight (slightly) on her front foot while she kicks her back leg during the fall forward into the floor. The nice thing about this concept is the same thing that helps you get upside down will make the turn happen, and the turn becomes easier when they forget the turn and concentrate on kicking their back leg. The big bonus is they can then concentrate on keeping their body in a the strongest handstand alignment/posture possible...... no, not the best possible for a roundoff, but the best possible position you can imagine for a handstand.... Make her forget she's doing a roundoff and beg her to use the handstand posture she needs to move efficiently through the support of her first hand and block off her second hand. The block from her second hand will help her finish her turn (so 1/4 going in and 1/4 coming out... mentally any way) and as a special prize the block will push her up off the floor into her snap down. I despise the term (snapdown) because it implies the action is best done when kicking both legs downward when the motion really should be a block upward that follows through into a scoop/hollow/dish/ tap action.
Sure she could be stronger, but the same problems would still plague her while she powers her way through a poorly done roundoff that has no rhythm and sense of awareness and balance.
The harder they turn, the harder they have to try to turn, and that just screws up the whole skill.
