I was really considering a Border Collie for awhile, because I wanted a running companion. However, from the pictures I've seen, they look a little too slender. I've got some little kids (the oldest is three) and I'd like a thick, sturdy dog that will be able to withstand any unintentional damage... I've pretty much narrowed my options down to a Bull Terrier, an Australian Shepherd or a Corgi.
Labrador should definately be on the list if you want a dog great with kids and not fragile that can run with you.
While I don't recommend letting kids tug and poke, I've never met a Lab that didn't love it.
I would consider a small Lab. Look at my post 33 - and look how short Dozer (the Black Lab) is standing against Patti's knees. If he didn't have all that show fat on, he wouldn't weigh much more than my girl.
Shelley is 20 1/2 inches at the shoulder, or 21 1/4 (depending on whether she was measured by a USDAA or an AKC judge!) and she weighs exactly 47 pounds as of last Friday. She is as tough as nails, has great endurance, and although she has that hot field trial temperament, she's very biddable and obedient. For example, she never pulls on a leash, ever (well, unless a squirrel runs by right under her nose).
I know some field trial dogs that are on the small size (Shelley's good friend Tori weighs 55 pounds and is no taller than Shelley - the difference in weight is due to her longer back (another field trial trait)) but your best bet for a small Labrador is to get a show dog with English breeding predominant (the English Labs are smaller than the American Labs) -- and don't over feed the dog!!!.
There are field trial Golden Retrievers that are small like Shelley - the show Goldens tend to be huge big dogs. Of course the show Goldens are the mellow ones. I thought about a Golden but couldn't deal with all the hair and the constant shedding.