Baby Harry, like 1-3 MAYBE 4. Point is, he's really young, Voldemort isn't back, and Severus had been tasked with watching over him. He constantly says he hates watching over the baby, says he doesn't want to at all, says he feels nothing for the boy.
Of course, being a spy, he has to attend death eater meetings. Per say one of the members found out about Harry, and now to "prove his loyalty" he has to bring him in. He thinks it's going to be fine, maybe a couple of harsh words, but nothing bad.
He's mistaken (of course) and has to watch as Harry is mistreated. Up to you to decide HOW mistreated. Maybe it's a couple of slaps, I prefer crucio.
Extra angst if Severus is 'invited' to join, and has to harm Harry.