There are several things going on here.
First, in a country where most people's basic nutritional needs are met, most kids are reasonably nice looking, say at primary level, also because children have features that make them good to care for, so big eyes, cuteness and so on.
I work at a university where lots of middle-class children go and most are good-looking these days, in that they have good nutrition, grow taller, hair care products are cheaper, they are young, slim (most are at this age), dress well, are into fitness/gym (this is a big thing for many young people) and so on. Anyone that gets acne goes to the doctor rather than just suffering on. Few are truly awful looking at this age- but most are not model-good looks, whatever their parents believe on here!
It is class related, and mumsnet has a distinctive demographic- if you get your kids teeth fixed, whitened, they stay slimish when young, have long swishy hair, or the boys go to the gym, they will look attractive- but so do the rest of their friends in the main.
My children are good-looking in this model, but not model good looks, just nice looking gorgeous young people, as indeed I was in my time.