I agree - experts say that before puberty is best.
I know its scary, but you can always approach the idea with your daughter and see what she says. Maybe she can start by putting together a letter and package of pictures for them - and they can do the same for her.
Then maybe you could talk to bmom on the phone and tell her what they told you. Then maybe have a couple of phone calls between all of you and THEN proceed to a visit.
I think as long as the grownups take on the stress of the relationship - and work to be respectful of each other, you are going in the right direction
Jen