It seems that there is not much of this offered in h/j land. There is an AWESOME eventing instensive riding camp for Jrs. at GMHA in Vermont. I think it is called the Jr. Horsemanship Camp or something. Top notch instruction all day long so if she is ok with trying some new things she might love it. Total immersion of all things horse, horsemanship and riding.
Stoneleigh Burnham in Greenfield, MA has a riding camp and that has more of a h/j vibe to it (although they do eventing too). It is the Bonnie Castle Camp.
The only other camps I can think of have good horse programs but are more of a traditional “summer camp” experience with swimming and archery and crafts and all that stuff. Camp Forest Acres in Fryburg (sp?) Maine, Camp Nashoba North - there are a bunch of those out there.