Just to preface this - I am all for children being outside (actually, I think it's wonderful) and have no issues with the normal garden noises of children playing outside whatsoever.
There are two privately-owned basement flats where I live, who access the back garden (which they share). There are neighbours in the house to the side of us who own the whole house, and the rest of where I live is shared accomodation...
Side neighbours have children, and yesterday one of them (who must be about 7) was in the garden shouting "Mummy" at the top of her voice, every 3 -7 minutes from around 8am, until lunchtime, and for another couple of hours in the afternoon. It was in the "Mummy come here" or "Mummy look at this" or "Mummy where are you, I want you here" type thing, and then getting louder and louder until the mother came out. It was loud; their other two children were playing outside too but they were just making "normal childhood playing noises"...
Our other side neighbours shouted at the child to "Please be quiet". Nothing more, nothing less. I have no idea if there were complaints at the house, but the kid went in late in the afternoon and all was fine.
Our downstairs neighbours had a barbecue last night - I wasn't invited, but they were in the back garden having normal adult-type chat, nothing boisterous, a little bit of music until 7:30 and then they moved inside and they all went out at about 21:00.
The Child was in the garden today at 7am. The same amount of screaming although it was "Mummy get out of bed", "Mummy come to the window" etc. At about 10am it stopped as the child went inside.
The mother saw me coming in off my bike today and has complained about the noise of the party last night. I wasn't there and I said that. She then complained about the complaint against her child (I think she meant when someone said "Please be quiet" and I said "Well compared to your other children playing in the garden, they were being very loud".
Turns out I am completely unreasonable. Am I?