It's salmonella, first of all.
Most likely source was food that was not handled safely. If the fridge is not below 40 degrees, if they eat leftovers over 3 days old, if cooked food is eaten after being left at room temp for over about 30 minutes, then these are the most likely sources. Thawing meat at room temp before cooking is another common source.
Some chickens do carry salmonella, but we are talking about one in 1000, last I read. Even if the kids handled the chickens, and the chickens were carriers, and they washed their hand afterwards, this is quite unlikely.
You'll have to do a Google search to resolve this, I imagine.