In reply to ConstaBern:
Has she already used an indoor climbing wall?
I recently took a 4 year old on his first slab climb out here in the French Alps. He had never climbed indoors apart from on the furniture. My main concern was him being able to lower off from a top rope. Lots of practice on lower levels before I allowed him to move higher.
I think its really important to find something he's not likely to get stuck on and something with a walk off at the top where one parent or friend can wait and encourage her from above. Personally I wouldn't pull a little one up, what's the point? Let her climb as far as she can and if she gets stuck, encourage her with your voice but not by pulling her and making her believe she climbed it. Lots of congrats regardless.
Sorry I can't help with location. I have been and climbed at Portland but my memory fails me regarding there being easy routes.