Medium sized dogs go to about 50 lbs. When we had small children, we were told to consider a medium to large dog because the larger the dog, the less likely they are to resort to biting or other aggressive behaviors to handle kids. Relatively speaking a larger dog will find it easier to just get up and walk away from the kids when he needs a time out rather than having to bite or snap to assert himself. Also, size is not determinative of whether a dog is good indoors or not. Some larger dogs are relative couch potatoes, while others need a lot of room.
We own bearded collies, petit basset griffon vendeens (PBGVs) and a border collie. Often with herding dogs, the males are a little more docile. Our first bearded collie decided I wasn't babysitting well enough when I turned to get a towel to get the one year old out of the tub. The four month old dog jumped into the tub and sat behind the kid so she couldn't fall. When the same daughter picked up the dogs foreleg by the hairs (it had to hurt) and I fussed at her, the dog just looked at me like, "we're playing, leave us alone." Our kids could crawl in and out of the dogs crates, it never mattered. Beardies are easily trainable.
They are long-haired dogs, but if they aren't shown, their hair is often trimmed short.
PBGVs are indestructible 35 pound french rabbit hounds. They love kids and they can be indoor/outdoor dogs. They are amazingly outgoing. They are not particularly good obedience dogs, but they are fun to live with.
People have recommended great resources. After narrowing it down to about three choices, go to a local dog show. You should be able to see the breed and talk to some breeders and owners.
Once you pick a breed, contact the breed rescue group (find the parent breed club on the American Kennel Club website
www.akc.org if you're interested in rescuing a pure bred or nearly so specimen of the breed. The parent club website will have breeder references also.