Wow, 18g strings and gut for young juniors, you must be in the 1%!
Young juniors and especially girls, should use at the most hybrid poly main and syn gut/multi cross.
My recommendation for 12 and younger girls is full multi or syn. gut. Soft strings for developing young bodies help prevent arm, shoulder and wrist injuries.
With full multi/syn., a full time junior player will have to restring every 2 to 3 weeks.
With my son, we used multi till he was strong enough to break them within 1 week. At that time, I went with poly main multi cross. That set up lasted almost 2 weeks, at which time it was time to restring.
With my daughter, we used full multi till she was about 16, at which time she was strong enough to handle a poly main multi cross.
Search the string section, you'll find that poly strings are over rated. Best use of poly is for players that are frequent string breakers.
If you have the time to string, it's a great money saver.