That's a new one...haven't heard that before. If you're in a non-exit row, then...well, you're not in an exit row. I've never seen any briefings or documentation that calls out that children cannot sit in front or behind an exit row.*

(You specifically asked "Can we sit with her in the row behind the exit row?" That should be fine. What can't happen is for one of you to be in the exit row while you have a child somewhere else on the plane.)