Terriers in general, I think, could be remarkably different from what you're used to with large breed dogs. Terriers can be real TERRORS, lol (my parents have a West Highland White Terrier - she is a handful). My grandparents adopted a Jack Russel puppy...Trixie..when I was in middle school, and they just couldn't cope with the energy. It was a massively poor decision on their part, and also a poor decision on whomever sold the dog to them. She was rehomed to someone with Jack Russel experience though, and it worked out in the end.
As for being good with kids, my only real experience with Jack Russels was Trixie, and I was probably 12 or 13 at the time. She never got nippy or anything, and I loved playing with her. My grandparents loved when I came to visit too, because I would run around the yard with her and wear her out a bit. She would jump, but I imagine that behavior can be corrected.