From personal experience, if you can find one for a decent price I would say get a yearling to an adult. Sure watching a hatchling grow up is fun, but I look at other aspects in the equation as well.
My Reasoning:
1. Young snakes tend to be very sensitive to handling. They stress out a lot easier than older snakes. Older snakes just kind of take it in stride.
2. Young snakes aren't as adaptable to a new environment if you're a first timer. Some things you have to learn as you go along, and with babies there's less leeway in this. Adults can adapt to whatever situation for the most part with less drastic issues. So if your temps aren't exactly right or something else is wrong, the adult won't be nearly as harmed by it as a younger snake might be.
3. Young snakes tend to be very flighty and nippy. At least all the one's I've have. And if you've got kids who're always going to want to touch it or hold it (maybe in not the right way most of the time) it could be disastrous for both the snake and the kids. If it rears back and bites, while it doesn't hurt all that much it's startling none the less..and invariably the child will drop it. An adult snake is MUCH easier to locate than a baby.
4. Feeding issues. I have never had a feeding issue with my adult snakes that couldn't be explained. My males will sometimes go off feed during mating season, most often not, but sometimes. My female doesn't eat right before she lays or sheds. But with my younger snakes, if something isn't just right you end up with a regurge or not wanting to feed entirely.
5. Risk of harm. If this is your first snake, I find them a lot tougher in the area of being handled. Sometimes kids don't know their own strength, and may inadvertently squeeze the snake too hard or some other thing. I just think adults are more resilient to this sort of treatment.
And honestly, I just think adults are more hardy in general. Once you've got an adult mastered, and you and the kids get the hang of their habits and requirements by all means go ahead and try your luck with a baby snake.
Hope that helps a little. =)
PS: I'm going to be posting an adult male snow up for sale sometime this weekend. If you're interested beforehand e-mail me and I'll talk to you about it.