Boxers fighting

Question
2 female boxers have got along until 2 weeks ago, they get in some serious fights. usually when kids are in yard and want to play with dogs. Any suggestions to stop this behavior.

Answer
You didn't give me any background info; their ages? Have they grown up together? Sisters? What?

If they got along and now do not, it sounds like an "alpha" competition and competition for playing with the kiddoes.

It is not uncommon for jealousy and wanting the people all for themselves and not share.

Buy a whistle, a loud, obnoxious one, and supervise the two playing with the kids in the yard, and the second the aggression begins, blow the whistle continuously, and separate them. I would leash them.

Verbally reprimand them, and take them inside and do not allow them to remain outside to play. This is a punishment.

Soon, they will associate the aggression to the whistle and verbal reprimand and not getting to play with their bad aggressive behavior.

You must supervise them until you have it under control.