The child seat is a sexual position performed between two men. It occurs when the men position themselves opposite each other; one lies on his stomach, and the other lies on his back, with their legs overlapping. The two men then bend their penii far enough to insert it in their partner's ass. They then move back and forth until orgasm, much like two women using a double ended dildo.
"Dude, you wanna do the child seat?"
"Nah, man. My ass is still pretty raw from last night."
"...Give me a rusty trombone, then."