@TaeFurey How has he broken his promise?
he can't control who turns up to be on the team.
he's supposedly an adult coaching a kids football team, he can't keep volunteering to do that, then dropping the team to suit his girlfriend, it's supposed to be a commitment to the kids.
You all sound very young, but you have kids, you need to behave like parents, adults.
you split up, THEN he started seeing this woman (doing nothing wrong) then he wants you back (hopefully he had the decency to tell her before you got back together?!?!) all that's happened now is her kid wants to play football - he can't control that! It's not like he's back in her bed.
he will be faithful or not, that's up to him, you can't keep him on a leash! & there's nothing to stop you doing as you planned, supporting him, the team & being sociable at the club.
she's some woman he saw when you were split up, she wasn't even someone he cheated on you with.
she had more reason to be upset than you.