1) No dog should knock your kids over when playing with them. No matter its size. Most larger dogs CAN knock over a toddler just with a tail swipe. You don't have toddlers, though.
2) Almost any dog (even the little ones) can pull a 3 or 5 year old over with a leash, if they aren't trained to walk on a leash properly. Kids those ages just aren't strong enough (or mature enough really) to properly manage a dog on a leash. Unless you are talking about some sort of teacup dog. Leashes get wrapped around legs and furniture or trees, etc... if the person holding it isn't cognizant enough of what is going on. 3 & 5 yr olds? Be prepared for some leash burns if you are planning to let them walk your dog (no matter the breed or size).
Multiple dogs? It can be good, or not. Just depends. It will be a lot more work for you in the beginning for sure. And I do mean YOU. You will have to train whatever dog(s) you get and your kids are not mature enough to help in the training process. They can fill water bowls, food dishes, etc... but actual training will need to be managed by an adult who can project authority. I have yet to come across a 3 or 5 yr old who can project sufficient authority as to train a dog.
Some dogs need a friend to keep them occupied. Some dogs are neurotic and can't be left alone. Etc... so a 2nd dog might be good for those types. But this is more of a personality than a breed thing, I think.
We have a GSD that we got when our children were 6 and 3. They all grew up together under my watchful eye (kids are still growing, lol). Wouldn't trade the dog for anything. Yes, she did a little herding nipping that she got over. But she doesn't knock kids over. She is very gentle. And she appears ferocious and like a terror to people approaching our house (she stays inside and watches out the windows). She DOES bark to alert me of any unusual activity/people. And rarely barks at "nothing". She recognizes the neighbors cars, etc.. and ignores them with interest. (watches, no barking). She ignores the daily newspaper delivery that comes at 4:00 a.m. The one thing that is routine that she does bark at is the trash/recycle pick up. I think she doesn't like them taking "our stuff". ;)
She does shed. But it isn't a nightmare. And she is quite a comfort when my husband isn't home for whatever reason. Even if it during the daytime, like if I need to let in a service person.
She has enough energy to go and go and go until we all conk out. But she mostly lays around sleeping if we are being couch potatoes and aren't being active ourselves.
She doesn't wander when she is outside. She stays right with us, or in our yard (usually on the porch or the grass next to the flower bed). She does not like being kenneled (boarding) and does tend to get depressed when we go on vacation without her. In that kind of situation, having a 2nd dog would probably be helpful. But she is 8 1/2 and introducing another dog at this point probably wouldn't have the same effect as if we had done it when she was much younger. And then, it would have been WAY more work and expense than I was willing/able to do at that time.
Keep in mind getting 2 of any breed, means twice the expense. Twice the heartworm, twice the flea preventative, twice the rabies and other annual vet bills, twice the food, twice the boarding costs, etc. And they can live a good long time. Most dogs' average life span reaches at LEAST 8 years, many live much longer... up into the early 'teens' for many breeds.
I'd suggest making a longer list of "desires" in what you are looking for. Not just "won't knock over the kids" but include what you DO want and other "not"s .... not yipping/yapping at every single thing (for example). And compare also what they are like not just in puppyhood, but as mature adult dogs as well. GSDs can be a lot of work as pups, but gosh they are SO worth it when they are past that first 18months....
Good luck with whatever you decide.
I know you want a smaller dog than a GSD. But size isn't everything. :)
ETA: tee hee... our dog even ignores the herd of chihuahuas next door. They have either 4 or 5(?) and are constantly yapping! I hear them everytime I walk outside. They are barking at ME (in my own yard). My dog totally ignores their noise. The dogs through the woods bark back and forth at each other (hunting dogs in kennels, on the dirt road through the woods behind our neighborhood), our dog just listens alertly, and ignores them. She doesn't participate in the cacophony.