I don't think 85 is at all high... in fact, it's kind of low. 100 is high, yes. For climbing hills, if you like to develop power and mash the hills, cadence will be a bit lower, but I really avoid going below 75 until I'm in my lowest gear and don't have any more choice.

What I'm seeing is that she's going from riding not at all to riding with a baby. Her joints are going to be taking the stress until her muscles are strong enough to deal with it. Can she either ride a trainer or find a sitter until she develops some leg strength?

Cleat placement and fore/aft saddle adjustment are as important as seat height in taking stress off the knees.