we've put in our will the choice of two families (my sisters and hubby's sisters), let them both know and then if the time ever comes the two families will decide who is in the best situation at the time to take on another child, with a moderator. we choose this path because, at the time, one couple didn't have any children and we didn't know how they'd be as parents, but the other couple were fantastic parents.
now - the fantastic parent couple have divorced and the no parent couple have a child and are fantastic parents, so probably about time we revisited the will ...
the hard decision was how to split the assets with hubby having three kids from a previous marriage who are nearly all adults, and junior was still a baby at the time - as the older kids won't need so much monetary assistance and still have inheritence/financial support from their mother and her husband, but with junior we're all she has/gets.