It doesn't concern your neighbours if it's earlier enough in the evening.
At the end of the day the law is up until 11pm (i think it's 11pm) it ok and only after where noise can be reported as anti-social.
Ultimatly considering their ages I doubt they are even out that late what 7-8pm? I hardly think that's late at all. I don't even have kids but this seems like an over-reaction to me on children playing outside, I bet they're the kind of neighbours that complain kids spend too much inside playing games in front of the TV too.
I don't know if your children and hit the frequency of bats or something but shutting the windows tends to drown out most noise so I can't imagine they are screeming to such a level the windows are shaking or something. I think if it's later at night 8pm onwards or something then sure you can start to say yeh it's be inconsiderate.
You can ask your kids to keep it down but kids will be kids at the end of the day.
Unless after all this we find out you bought them a drum set, amplifier and a guitar and they're out their screaming death metal... then op... you're a dick