You can use fully poly with your daughter, however people need to understand the top 3 components of a junior stringjob - Type of string, gauge, and tension.
Your daughter SHOULD be using full poly as no other string will allow her to produce as much spin. This is paramount.
Many people do not understand the proper way to make adjustments when stringing poly for juniors. The next most important detail is the gauge (thickness). Unless your daughter breaks strings every day and hits as hard as a college player, she should stick to 17 or 18g poly (the thinner the better as thinner produces more spin). This means the string is inherently less stiff, allowing her arm to receive less shock upon striking the ball.
The third detail us tension, which often gets overlooked. Poly is a naturally stiff substance, so make sure to string significantly looser. Ideally we're looking for something in the 40's. This will mean your daughters strokes fly farther, which means she has to (gets to) put more spin on the ball, all while saving her arm.
If you have any more questions, feel free to email me at mprevidi @ gmail.com . I am the head stringer for the Solinco stringing team and string numerous national tournaments for juniors every year. It's time to start dispelling the myths that poly is bad......it's all in how you string it!!