Socialisation is the thing which ALWAYS gets thrown at HEers as the reason not to do it.
HE parents do not keep their children locked in cupboards. At least, none that I know. Like I said before, there are active HE groups almost everywhere in the country. Take a random local area: perhaps an under-7s learning group once a week, an under-7 playgroup once a month and under-5 meet-ups in people's houses once a week, an all ages social get together at an adventure playground once a week, ice skating once a week, swimming once a week, climbing wall once a week, and once a week a whole bunch get together for foreign language classes. Pottery, needlework... it's all happening.
That's before you take into consideration the families who post on the local list that they are going on a nature walk at X on tuesday, and everyone is welcome to join, or that they have set up a tudor day at a local museum cost £5, only 25 places available. And of course families have informal get togethers all the time which no-one knows about because they aren't posted on a list anywhere.
Did I mention the summer camps? As far as I can see, half the HEers in the country spend most of June and July camping in fields at HE festivals (HESFES is the famous one) which are a big mix of social and learning and having a blast.
It's a whole glorious subculture, and it's growing FAST.
HE children still make friends at the park or at cubs or swimming or sunday school or wherever.
The difference for HE kids is that if they don't get on with someone, they don't have to spend 6 hours a day with them 5 days a week...
So, what precisely is it about the social milieu for HEers which bothers you? It's different, for sure, because it's not being in that bizarre and artificial environment of being in a room full of 30 people the same age (name a single situation in adult life where that would happen). But rich and varied and as social as the child wants - yeah baby.