We've found that how open the friendship can be totally depends on the other family. For instance, my dd11 has a friend whom she has known since they were 2-3 years old. This relationship just got harder and harder as the girls got older. I believe that a lot of it had to do with the other child's parent having some insecurities and being in denial that there was a difference between our children. Everytime something obvious made it clear that they were not of the same intellect, it created a blow up or the other parent getting really hurt & upset.
I have another friend whose dd is close to dd11's age and, again, there is some degree of a difference in terms of the children's abilities. However, this other child is probably close to gifted or at least somewhat above average. Her mother also has no illusions that the girls are of the same ability and doesn't seem to have any insecurities about it. She accepts my dd for who she is and celebrates her own child's strengths without comparing the two constantly. This has, obviously, been a much easier friendship to maintain.
The biggest problem in the first instance has been since dd has been older (5+) and has been very aware of the other parent comparing them. Dd was very uncomfortable with being asked to read things to the other mom when she was over playing with the other child or with being asked at "what level" she was doing various things. I tried very hard not to get drawn into getting upset when the comparisons and drama was directed at me, but it became a significant problem when it started to be directed to dd.
