We have a Border Terrier, she's the loveliest little dog, very friendly towards people, and her and DS, age 3, get on brilliantly. She can be snappy with other dogs she meets, but it's all just noise (a bit like she's saying 'who are you looking at?!')
We also have a Jack Russell, he's completely different personality wise; he's clingy to me and can be quite grumpy, he is old though. He was snappy with other dogs in his younger days but I don't think he can be bothered any longer!
We've previously owned a rescue dog (a terrier cross) they can make great pets but you often don't know their background. That can lead to issues with training, behaviour and unknown health problems, as in our dog's case, sadly.
So terriers are great little dogs, full of personality, love people, can take lots or little exercise, but they are headstrong and can be snappy with other dogs (small dog syndrome). Contrary to what people will tell you, they are fine with their own cats, but they will chase stranger cats and I would not trust them with small furries. I would not let a child walk either of mine, but I don't think children should be walking dogs anyway (in case they meet a terrier
)
We had labs when we were young, they were great family pets, good with people, children and other pets. No aggression and can be as active or lazy as you want them to be.
In my extended family their are a whippet and a lurcher X. The whippet is friendly but loves to run (think lots of off lead exercise), and is a bit of a scaredy dog. The lurcher is a rescue dog and again loves to run but is terrible at coming back.
All dogs are different, study breed traits and meet the pups parents when you decide to see what their temperaments are like. Be prepared to do all the work yourselves as children of all ages lose interest quickly. A dog is something the whole household must want in their lives, and is very much part of the family.