Why don't you guys ask an MRT or an actual teaching pro who has experience with this stuff?
Polys are bad after the first few days. Then they don't break. They stay in there. Kids swing hard, get control, but also develop some pain.
Gut is a good option. It stays fresh for a long time. Holds tension well, too. It is good until the day it breaks. You can combine it with poly if you want (mains or crosses). You can combine it with a cheap synthetic, too, to save some money.
The poly craze is crazy for young kids. I'd much rather a kid develop with the 'disadvantage' of a multi or a natural gut stringbed until he or she can safely use poly in the teen years.