You can have Salmonella infection and be asymptomatic, but as you said, young children and pregnant women are both high risk for serious illness. When my children were little, I limited their handling of the birds and was militant about handwashing. You might consider not having the kids handle the chickens until they are at least 5 years old, for example, and be careful about not having items from the coop inside the house, especially in the kitchen.
I'm not aware of any way you can know whether your flock has Salmonella, and honestly, I would treat them all as if they did. I understand that Salmonella is in the ovaduct, so when the eggs are laid, they are already contaminated. I have asked before how long birds can carry Salmonella and have not had a good answer to this. We know in people, they can become chronic carriers for 6 months or more- I don't know whether this is the case with birds as well or not. I think it is better to be cautious and have exceptional hygiene where chickens and people intersect.