DD is in Year 6, and out of the 45 in her year, I know there are 6 only children (including DD), two more who might be only children (don’t know the families well enough), and 4 children who are one of 3. The rest (so 33-35 children) have one sibling. The siblings are generally a few years younger, so there’s no one with a baby or toddler age younger child now.
This is a school in a fairly affluent area of a northern city, though the catchment area includes some more deprived areas, and is mainly white British/ white European.
I had DD when I was 38 and couldn’t have a second due to age and health issues; and of the other only children, I know that one mum is the same age as me so could have had similar issues, one mum split up with her DH when her DC was a toddler and hasn’t met anyone else, and one DC was adopted.
Two children does seem to be the norm among my friends and wider family, with three children being unusual and indicating being fairly well-off (in the context of bigger houses, nicer cars, extra-curricular activities, a one holiday abroad - not private school and multiple holidays per year).
One of the mums I met at a baby group went on to have three more children - she is a SAHM and her husband has a well-paying job. She said she did get a few surprised comments about baby number three, and about baby number four. I’m not sure I could have coped with four, but I do envy her.