No she can definitely exercise! It's not really a question of waiting until her blood sugars are playing ball, it's more about discovering what impacts different activities and foods have on her and you both (as a team) WILL be able to manage them.
The exercise thing is quite complicated. It is absolutely a great tool for helping to manage blood sugars, because it really increases insulin sensitivity - so you're right in saying that exercise can lower your sugars.
However, once it's already too high to start with, things flip and it causes sugars to go higher. Because you are exercising you need more energy to power your muscles, but your body cannot access the energy from the excess sugar in the bloodstream - so it sot of gets a bit confused and releases more stored glucose from the liver in hopes that will be an energy source, as it's trying to fuel itself.
But there's not enough insulin there, so this sugar is just adding to the excess that's already there - and the numbers go up.
Also, the type of exercise you do will have differing impacts on sugars too. Long slow steady cardio type stuff will burn off sugar steadily - but for example a quick sharp burst can cause the liver to release some stored glucose too. So you get a spike.
Weights for example (not that a child will be lifting weights! but may be doing activities that have similar patterns of muscle use in some sports?) will give me a small spike, so I do weights first without reducing the basal on my pump - and then run with the basal reduced.
Watch for hypos after exercise or activity, because sugars can start to fall hours after. That might sound scary but honestly you will come to know what works for your child, and you can use exercise to your advantage - for example, when I do weights I'll see steadier sugars for about 24 hours after my workout.