I would do nothing. It's over. Let it go. Even during the party, any action to be taken would be the responsibility of the niece whose neighbor this was. The person who lives in the neighborhood has to continue living next door and so must be the one who decides if any action is to be taken.
If this were my neighbor, I would've gone next door, taken the boys home the first time they came over, and told the mother that this was a private party and that her children were not invited because they are not friends or relatives of the birthday boy. I'd word it as diplomatically as I could at the time but I would be very clear that they were not a part of the party.
If this caused bad feelings, so be it. One has to have boundaries! And one has to stand up for those boundaries in order to have a livable neighborhood. Not speaking up will cause your niece to have bad feelings and as you saw put a damper on the party. I strongly believe that taking care of ourselves and our family comes first over trying to make anyone else happy.
Now that it's over, if I was your niece, I might still talk with the neighbor. to let her that her son's came back over and act as if the mother didn't know it. Give the mother an out but let her know that you won't accept this sort of lack of consideration again.
Sounds like she doesn't know the neighbor. If that's the case I'd make a point of getting on casual but friendly terms with her so that I'd be more comfortable handling any other situations that might arise.
Only the niece who lives next door should be the one to say something to the neighbor. The neighbor would rightfully dismiss anything said by a stranger and be upset with your niece at the same time because she would think that she was allowing the stranger to interfere, not having "guts" enough to talk directly with her.