Hydro at that age would be held with the instructor in the pool. Depends on if it was a group session or one on one if you'd be needed in the water as well. But definitely a worthwhile question to ask. Depending on Froggy's needs, the therapist may also give you some hydro exercises to do between sessions, so you may only need to pay for one therapy session a week, but then you could take her for another 1-2 sessions if required. This would mean hoping in the pool during the initial sessions so you can get a feel for what is required.
Hydro is normally run by a physio or exercise physiologist and sessions are generally 30-40 minutes. Kids can't cope for much longer in the pool, due to limited concentration but also regulation of body temp.
Also, therapists should make the session heaps of fun so the child doesn't really see themselves as working hard. Playing heaps of games etc.
If she water confident already, this would make a major difference to what exercises can be done in the water. Hope this helps smile