Labs are great dogs, i've got 5 of the big 'ol galoots (and 5 pups just now old enough for their new homes). My dogs are pretty well trained in obedience, one is a superb bird dog. Even though they behave well for me, I know that they easily give in to temptation...I'm making sure my coop is secure against them before I put the chickens out in it.
As long as I am present, I know that I can control the dogs. See, my dogs and cats are great pals, eat together, sleep together..the whole shebang. But, even though the dogs like the cats, if the cat runs from the dog, the dog will chase it. If the cat doesn't run, the dog doesn't chase. It's a game to the dogs, which is fine, but they get caught up and I have recently lost a cat to 'the game' because the dog played too rough. If I call the dog back, it will stop chasing, but if i'm not there to supervise, well, you know...
I seriously doubt the chickens would not run from a charging Lab, so..no doggies with the chickies.
My point is that with Labs, even well trained Labs, you have to keep in mind that they're 'puppies' at least mentally until at least 2 years of age. I'd keep up with the supervised 'visits', it's sure to help. Just don't give over complete trust because above all, Labs love to play.