Have a look at Head RIP Control. It's a rather soft multi that plays more like a poly than a synth gut - lower powered, good access to spin, and I find it more durable than synth guts. Also holds tension well and it's cheap if she breaks strings a lot.
My son is a big 9 year old and does group clinics with older kids of 12-14. Some of them used RPM when it became popular and some developed elbow issues, although could be technique related. My son wanted "those cool black strings", so I strung him up some black RIP control at 50lbs and he likes it.
Pro Kennex 7G, Head Rip Control 17 @ 58lbs, rubber band dampener, Tourna Grip.