I have 3DC, DS1 is 7, DS2 is 4 and DD is 8 weeks. We are lucky enough to have an 80ft garden at the end of which we have a trampoline and climbing frame. This easter hols we have been out at parks etc most of the week but came home this afternoon at 2pm with my SIL and her 3 DCs aged 4,3 and 7 months. They all played in the garden until 4pm when we let the older ones watch a film inside. At 5.30 a man came to the door and talked to SIL while I was BFing DD upstairs, I could hear he was annoyed about something, so I stopped and went to the door.
I asked how I could help as I lived here not SIL, after angrily muttering about having to repeat himself he told me he wanted to complain about the children's noise! He arrived home from to find his wife very upset. Apparently the children screamed all afternoon
(they didn't, there was a couple of tantrums which were resolved within 5 mins and a water fight at one point with some shouting). Apparently this noise disturbs their dog and makes it bark which in turn upset the wife??? 
I explained we had visitors today and he said well its happened before. I guess we do sometimes have BBQ's with friends and their kids, but never going on too late. I said well they are young children and I didn't think there was any problem with playing in their own garden. I also pointed its not our fault that they have a neurotic dog and also mentioned that there are lots of children in the surrounding gardens at which point he said "yes, you are not the only people I've spoken to". My SIL said she didn't think they were being too loud, he replied "yes well maybe you are used to the noise but our children have grown up and left home"
I was polite to him and said If the boys screamed incessantly (not likely to happen!) then of course I would tell them to be stop, but explained that I would not be able to or want to stop my children enjoying themselves in their own garden. My boys are always in bed by 7.30 and are polite well behaved kids who just like to be boisterous and have fun in their own space!
AIBU? What should I have said? Time for
I think