Enough is enough. For the safety of your family, other members of the neighborhood, & the dogs themselves, they need to be contained. I am sure there are also ordinances stating such in your community.
You have talked to the mother on 2 different occasions, and she is obviously not taking this as seriously as you are.
911 would probably be escalating this too far, as the dogs are not posing a truly vicious threat. However, a call to animal control would be 100% appropriate. And no, you do not need to contact the mother again. She has been given her fair warning (twice).
Call now, & call again EVERY time the dogs get loose, whether the family comes to collect them or not. Make your own documentations of these occurrences as well.
If the family contacts you & asks why you are calling animal control on them, you can explain calmly that you had tried keeping the city out of it, but your dogs were continuing to run loose in the neighborhood & you were concerned for your son's safety, based on their behavior.
If they have a fenced in yard, it -shouldn't- be difficult for them to keep the dogs contained. And if they don't have a fenced in yard, I have a feeling one will be put up rather quickly. =-)
P.S. I say all of this as a dog owner myself. If I weren't being a responsible owner, I would expect & deserve to have animal control called on my dogs, & I would make darn sure that it only ever had to happen once.
Hopefully, once is all you'll need as well! T. =-)