They take blood from a vein the same
way as they do for adults.
Usually, the child sits in
your lap and you restrain him while
they take the blood.
It's harder on the parent than on the child...
trust me.
enigma
I understand that they won't test a child before 18 months. I had my 3 yo daughter tested, and they just took a small amount of blood. I held her. She is a very active child, too. I prepared her for it by telling her the dr would take some blood from her arm (she's a bit older, so I don't know how much you could explain.). My daughter was fine and stayed still. She was curious, which helped.
I had my son tested at 18 months at the lab at our local children's hospital. They were wonderful with him and he didn't get upset at all!