Not a dog expert at all, but think the comments about all types of dogs 'can be dangerous when reared badly' is correct.
I have only three experiences of Jack Russells. My ex next door neighbour had one, and he lived between my house and theirs ...loved him to bits, never showed any type of aggression even though there has always been plenty of children around. The only times he got aggressive was when he spotted my last cat ... but believe me he got the worst of it ...
Amazingly, my ex. neighbours have now rented their house and, guess what, the new tenants have just acquired a puppy Jack Russell. He is a little naughty at the moment .. keeps running into my garden, but comes back wagging his tail when called.
Just taken him back early on .. came to me when I shouted his name, picked him up, gave me a saliva beauty face treatment and returned him ... he is adorable.
My friend had a long haired one and he was a different kettle of fish ... when I went to visit, would jump up at you for 10 minutes and quite nippy .. acquired a few bruises. However, my friend looked after her grandson from birth quite a lot and I have babysat on a few occasions ... again no aggression to the baby. Unfortunately, he did attack and kill her next door neighbour's cat, so she had to have him put down as was told my the vet that once they got the smell of a kill, may not stop. R.I.P. Eric.
I have a canine colleague at work, who is a labrador, and find that he loves the company of other dogs. Why don't you take him for a couple of days Tintin to see how he behaves.
Sorry to witter on, but would not say no to having this breed of dog.