Where do these attach? Socks?

Yes. Attach two on each side of the front part of your shirt and clip it on the inside part of your socks. Clip the other two on the back portion of your shirt and clip it on the outside of your socks. I found that if they're attached on the front of you socks they'll rub on your leg as you walk.

how do the one size style work?

Basically you clip them to your shirt then to your socks if they are too loose then you need to adjust them shorter. If they are too short you need to adjust it so it's longer but it needs to be tight enough to pull your shirt down still

Will these work for boys? Ages 7 and under?

Depends more on the size of the boy than the ageI doubt this particular set would work for a preteen or teen under say 150 lbs unless they have well built thighs

What’s the size around thigh? elastic? velcro? clip? i’m a skinny guy and don’t won’t it lose around my leg because that defeats the purpose.

These don't go around your thigh. They have a clip on each end. One attaches to the bottom of your shirt and the other end clips to the top of your sock.