I just watched The Boy in the Striped Pajamas (great movie, never got to read the book though) and when Bruno was meeting with Shmuel (8 year old boy in the camp) got me thinking what would small children do in the camps and what would happen to them. In the movie there were a couple of times Shmuel would exit the scene with a wheelbarrow, but the idea of a small child doing physical labor seems impractical.
Depends on what camp. Children were extremely useful in labor camps for working on machinery that adults would have difficulty reaching into (similar to arguments in the late 19th century for why child labor was needed). Children do actually exert a significantly high amount of work and effort with minimal food intake and space usage.
However at straight up death camps like Auschwitz, they were immediately disposed of as worthless. According to eye witnesses babies were occasionally immediately tossed into open trenches and burned with flamethrowers. Stories of bayoneting babies and children were not uncommon. At the death camps possible life or immediate death was a matter of if you looked 16ish to a person evaluating you when you got off the train. Others would be gassed along with the elderly and weak.