I have checked on a few breeder rehomes. It seemed to be a luck of the draw. Some may retire a bitch/ dog from their breeding program or may get one returned. Most breeders are pretty thorough on their home screenings, so I have not come across high amounts of them.
I would start talking to some good, local breeders and ask them to keep an eye out. Many breeders chat at shows and the word will get out. It may take a little time. Ex.I was talking to a breeder that is part of our Newf club in April and she knew of a bitch that was getting retired from showing/ breeding from another breeder.
As per the male/ female thing... your probably going to get a variety of answers to that. Most have suggested one of each, pending they are altered. You can have issues with either 2 males or 2 female of close age fighting for alpha in the house. I had 2 females, prior to Kayra's passing last year. I made sure there was a good spread of age between them and Sadie came in the house as a pup. She grew up knowing Kayra was the alpha. There was no question of pack order in the house.
There was a post on her a while back on male vs female.
http://www.newf.net/Forums/showthread.php?t=93923&highlight=male+female Some say males are more laid back than females. Some of the females... they have the name bitch for a reason. However, that being said the concensus seems to be that personality trait are more relative to the indivudual dog rather than a general as per sex. My Sadie is pretty laid back.
If you goto a breeder you can explain the personality of current dog and they could probably be the best judge of a match. Many of them are wonderful at matching the perfect dog with the perfect household.
Children.. I raised a pup with a 2 and 4 year old. It depends the amount of time you have. They have all grown up together. I was a little afraid of bringing an adult in not knowing the history. Having the pup and the kids grow up has been wonderful. They are best buddies now.
I hope this is of assistance