In reply to Scarab9:
My own daughter was outside on rocks from crawling (bum-shuffling!) age onwards. I bought her a harness when she was 3 and a half as she was starting to solo routes. She likes indoors, but also struggles with the reach between holds. She generally asks me to pull the rope if she genuinely can't reach. She also enjoys bouldering, where she can be more creative with holds.
I took my step daughter out from the age of 4/5, and although her father had reservations, she did enjoy it and still does. So ulitmately no issues, but care should be taken not to tread on the father's toes really.
Good luck, give a a try and it would be nice to get the father's blessing, but only if your small borrowed girl is enjoying herself! I'm sure she will, coz climbing is fun.