This summer, we brought home our second GSD (at twelve-weeks-old) when my son was a little over one year. So far, things have been going great, and I couldn't be more pleased with how the dogs interact with the baby. That said, having large, powerful dogs is a big commitment--even if you don't have little kids in the house! GSDs need training, exercise, structure, and boundaries, as I'm sure you know if you've had working breeds in the past, and if you've been lurking on this site.
How old is the puppy you are getting? I'd be cautious introducing an adolescent puppy or a grown dog to small children. I'm somewhat relaxed with eight-week-old puppies. As far as introductions, I'd probably just put the puppy in a travel crate in the back yard, let the puppy sniff the kids through the grate for a little while, and then open the door. I wouldn't let the kids crowd or overwhelm the puppy, and I'd supervise to ensure proper interactions (no grabbing ears or tails, ect). I'd probably try to keep things calm--minimize yelling, jumping, running, squealing, ect. Be prepared, and perhaps prepare your kids, for a mouthy puppy. Most GSD puppies play-bite A LOT, and sometimes people mistake this for aggression. It seems like it will last forever, but it won't. Keep a toy nearby to direct the puppy away from little hands and fingers.
I had to chuckle when someone mentioned keeping the kids' toys away from the puppy. That has been pretty challenging for us, as the baby's favorite activity is chucking things out of his playpen. We have lost a few stuffed animals over the last year (they just look too similar to the dogs' stuffies). At six months, our pup has pretty much learned to leave the baby things alone. The baby, however, still tries to play with the dog toys.
I don't have a toddler or a preschooler yet, but the dogs have been gentle and appropriate with my very active crawler. I was prepared to use baby gates much more frequently than I do. When the baby is out crawling, the dogs will follow behind him quietly (I think they are supervising), or they will move out of the way and lie down elsewhere, or completely ignore him. Still, I'd recommend crate training so you can keep the pup safely out of the way when you or your kids need a time out. I put the dogs in the backyard, for instance, when I'm feeding the baby. The baby LOVES to toss food down to the dogs, or hold his sticky fingers down to be licked. YUCK.
I think one of the best things you can do is to research breeders and the different lines of GSDs. The different types not only have different appearances, but may also have different proclivities and temperaments. GSDs are great whatever "flavor" you prefer, just make sure you find a breeder who can actually demonstrate that they are breeding for health and temperament. Temperament is doubly important when you have small kids.
Secondly, I would recommend investing time and effort in socialization, training, and age-appropriate exercise for your puppy. I would enroll your pup in obedience classes. Find a good and balanced trainer. I use mostly positive training, but a positive-only trainer probably is not the best fit for working breeds. I would also suggest you take more than one class. I personally keep my dogs in classes for the first year or two. I am not a trainer, breeder, or any kind of expert. But I know how to raise a good puppy, and I know how to train basic obedience. Still, I go to classes because it's great for socialization. My facility usually has several other classes going on at the same time. It's noisy. There are many dogs but they are fairly well controlled (unlike at a dog park, for instance), and there are a million distractions. When you're out in public with the kids, you don't want the GSD to be hauling you everywhere, lunging and barking at other people or dogs. Classes are a great way to help your dogs learn how to behave in public.
Lastly, make sure the puppy is getting enough exercise (appropriate for his/her age, course). It's going to be a lot harder to keep your puppy from tearing up the house, jumping and running around like a banshee, and knocking over the toddler, if he doesn't have a good outlet for his energy. When my GSD, Asher, was a teenager and acting like a jerk, I often realized it was because I had slacked off on his exercise that day.
Congratulations on your soon-to-be new puppy and your decision to add GSD to your family. If you put in the time and effort, (and it IS work at times), I don't think you'll be sorry!