Hi BearsMama - do you lay awake and night and dream of doggies?
I had a Lab for 12 years. He was the most wonderful, gentle, friendly, smart dog I've ever met. He adored the children - when we brought out twins home from the hospital he literally grinned from ear to ear and wagged his tail for hours: "Hooray! We had puppies!" The babies could pull his hair out by the handful, and he would cry if I tried to isolate him for his own protection. I'll bet he understood a hundred or more words (he learned to spell W A L K). He would walk across hot coals for me if I asked him to. There never was a more loyal dog.
That said, when it came time for us to get another dog, we did NOT get a lab. They don't really reach adulthood until they are about 3 - and that's a pretty huge, energetic, rambuctious teenager to have in the house! Ours got at least an hour of exercise (frisbee or tennis ball) every day, 30 minutes before we left for work and more in the evening. Even with regular brushing, his double coat meant that tehre was dog hair EVERYWHERE. He was big for a lab, and we had to get a van, just so we could take him with us when we went out of town.
When he was younger, he was a runner. If the door wasn't latched, or if we were outside and didn't watch him for a few minutes, he would be long gone.
Even when he was tired out, he took up a lot of room in the house, as Joanna said. He wasn't quite as hyper as the dog she described, but he didn't miss much. No matter where we were he would be laying at (or on) our feet.
Labs are definitely high maintenance, because of the amount of exercise they require. In your case, with small kids, I think it might be hard for you to carve out an hour or more a day to devote to playing hard with the dog (ours was 5 years old when the babies were born, so it wasn't as bad). The problem with adopting an older lab is that they don't have a real long life span.
I think Labs are the greatest family dogs in the world - but I don't think it's the right dog for you. Think seriously about waves of dog hair drifting across the kitchen floor! It would have helped if we had brushed him more often, but that takes time too.
When is the "right" time to get a dog? We got a puppy, and typically puppies take a lot more time in the beginning. We waited until our sons were 7 years old, so that DH and I could devote 90 minutes or more each day to the dog, without taking a lot away from the children. The 90 minutes isn't all at once - it's 5 minutes out of 20, when he needs to go outside; it's 10 minutes of training here and there throughout the day; it's in the middle of dinner when you catch him chewing on a chair leg.
An adopted dog will probably be house trained, and more polite, but will still require a fair amount of time and energy to help him adjust to his new home.