I have used pretty much every brand of gut out there, from Global Gut to Babolat VS.
I really notice very little difference among gut strings. Once they break in, they have very very similar playing characteristics. So save money and go with the cheapest you can find (within reason, some REALLY cheap guts break during stringing).
When I got the first "real" racquet for my daughter (26" Prestige), I cut out the synthetic it came with and strung it with Global Gut. That job lasted 2 years (we only play indoors, which certainly helps). She now has a 27" Pure Drive Lite now and I have that strung with Tonic and it really feels sweet. I am seriously thinking about switching myself to Pure Drive, just because of how it feels.
All of my racquets are hybrids with Klip natural mains with RPM blast and Luxilon crosses. They last pretty long time, much longer than a full gut job for me.
Klip Legend goes for $26 for a full pack and RPM and Luxilons are around $17. And I do all of my stringing with a Gamma dropweight, so it costs about $22/string job. But with this set up, I can go till it breaks and it will still provide comfort while holding overall tension okay. It lasts at least 50% longer than a hybrid with synthetics and the better feel really makes up for the price difference.
Only thing I implore with juniors is - it is fine to use a full bed of poly, but for your child's sakes cut it out after 6 to 8 hours. Otherwise you are just asking for arm problems. I just shudder at all these kids at the club with full poly string jobs with stiff Babolats, because I know most of them will go till they break.