Jackson is okay with most kids but it did not come naturally. He's always been great with my siblings (now 7 1/2 and 3 1/2) but strange kids would make him a bit more nervous. Now, he's actually REALLY good about kids. When children want to pet him out in public (and most DO ask, actually, which I appreciate), I tell them how to approach him and I usually carry treats in my pocket so they can give him one if they want. I started slow... I started taking him to my cousins little league games where lots of kids were present, we'd sit near the park where a lot of kids were on the jungle gyms, etc, and I'd give him a few treats just for watching them. Then we would start getting closer. I just always tried to make a positive association with them. He's a really shy dog so you just have to take things slow with him. He changed soo much within a year though. Now he will willingly go up to a child (probably cuz he thinks they have treats, LOL) and enjoy a nice pat on the head.
As for big dogs, the same thing. I just started with dogs I knew, like my grandma's Lab, really gentle and really good with him, and I think he just learned they can be okay. I try not to allow him in situations that will make a negative impact on him because with him, it will stick w/ him FOREVER, he does not forget things, haha.
This is pretty much how he acts with ANY kid who is willing to give him a treat, LOL