There are a couple of places in the US that offer combined programs (basically a BS + MD in 6 years or so). I didn't know I wanted to go into medicine at the time, so it wouldn't have mattered for me.
I LOVED undergrad. I had a great time, and even though I worked hard and studied, I really wouldn't trade the experience for anything. I also took 2 years off between med school and undergrad, which was a sanity saver. Even as good a student as I was, as hard as I worked, med school was far beyond undergrad as far as commitment and difficulty.
Children are sort of a moot point for me, but if I had wanted them, I could have done it. I thought about it pretty seriously, but decided against it after joining here. I think that if you want something bad enough, you'll find a way to do it. The others will have to comment on that.