I love GSDs, as far as I'm concerned there are dogs & there are GSDs.
I have 2 working line gsds, a male(3yrs) & a female(7yrs). I also have had GSDs while my children were growing up (my youngest is now 11yrs old).
When GSDs are puppies they are seriously full on. They need a really firm but fair upbringing - which means every single member of the family on the same page - i.e. "down" means down, not get down, lie down, do it when you feel like down - command words need to be decided on, used & followed through by every single family member(impossible for little children to remember to do) as well as lots of exercise - every single day, through rain/shine/thunder/blizzards. Depending on the individual dog(& owner) - they can mouth/play bite quite hard right up into adulthood.
Your children are still very young & I'm assuming not used to doggy behaviours - any puppy, let alone a GSD, is really going to be a nightmare.
Puppies play with their mouths, children then scream & run when they get nipped, the puppy thinks it's playtime ( it is for the pup) & does it even more.
The children end up terrified, the puppy ends up left by itself in the kitchen(where it will proceed to eat your units/empty the rubbish-then eat it & vomit it back up all over the floor/tip it's water bucket over/ nick food off of the worktops) or in the garden(where it will dig up your flowers/plants/lawn) and then eventually be left in its crate, as that will be deemed the only "safe" place it can be left. It will then grow up into a completely untrained, out of control dog that will end up in rescue.
I've had dogs - mainly mixed breed adult rescues - all of my adult life & even I found my first GSD seriously hard work, when you add children into the mix it's a whole different ball game again.
Personally, as your children are so young, I would leave getting a puppy - regardless of the breed - until they are older. I've had 3 children myself as well as puppies over the years, so know how difficult the 2 together can be.
If you aren't prepared to wait until then, I would look at getting an adult dog - as it will be your first dog - not a GSD, but an "easier" breed/cross, perhaps an older rescue, nice & calm, bomb proof that takes everything in its' stride. I think Guide Dogs for the Blind have a waiting list for their retired dogs - something along these lines would be perfect for your family to get the necessary experience to eventually be able to bring a GSD puppy into your household and raise & train it to be a well behaved adult & family member.