I guess the issue is ... the kids *don't* always come home in socks. For a while, we had a major shortage of socks and underwear for this reason.
Finally, they hit a turning point where the kids were mostly appropriately dressed.
They have pairs of children, rather than children. My son shares a dresser with his wife's 6yo, who is much bigger than my almost 8yo. So my son comes home in underwear size 10/12, rather than 6-8. When he wear socks home, they're often men's socks rather than boys' socks (both boys, actually, also my almost 6).
My daughter is the same size as her stepsister, approximately, so their sharing a dresser is not a problem. But my sons for a long time came home in clothes 1-2 sizes too big for them. Finally, the boys are old enough to insist that their clothes fit. Now their dad has told them they're not allowed to wear sweatpants to his house or he'll burn them in the fireplace (a few months ago I got rid of all the jeans they refuse to wear and replaced them with the soft pants they prefer). I've sent things like socks, underwear, bike helmets, etc. over there when they've had shortages. Extra clothes in the appropriate size. Etc.
A few weeks ago, he called me about the socks. I said I'd try to be more aware of it, mentioned the problems with the underwear, and thanked him for bringing him up. They've worn socks since, mostly. This past Friday, we house sat for my mother and I forgot to pack socks, thought, "No problem. I'll put them in the socks from their dad's house" (They were going back the following morning for another overnight). They *had no socks* when they came home, so I sent them back with no socks, as I had none to send. He and his wife have sent my husband and I approx. 8 emails about socks since then, insisting the boys wore socks home, insisting I'm a bad mother because the kids don't wear socks.
The boys wore new, specially marked socks Wednesday, and my dd and I had a conversation about socks. She said, "I don't wear socks at dad's house; he doesn't care if I wear socks." She was polite and respectful and I said, ok, and off she went to school. So I got an email about THAT.
And it's not (I think) about socks, as my kids have never routinely worn socks at either house. I really feel that it's a control issue. Certainly they follow his rules at his house, but if in the hurry to leave on Wednesday morning they forget their socks, then I might expect a brief comment at pickup "The kids didn't have socks; can you please try to double check them Weds. mornings?" and not this slew of emails or heavy pressure that stresses out the kids.