Parents who have kids playing tennis competitively need to make a decision as to whether or not they really want their child to play, and if so, understand that strings are going to break. Period. There is no way around it. Poly loses tension incredibly fast and dies even faster, even though it is more durable than a multi. Tension is not going to change any of the aforementioned.
You have 4 choices:
Poly (hard but durable)
Multi (comfortable but breaks quick)
Buy a stringing machine to save some money, or
Head Stringer @ the LTC, Babolat Star 4 Stringer