Noir didn't know why he had to watch Homelander’s kid. It's not like Noir was great with kids. Homelander had just rushed in, saying something about needing Noir to watch you while he did something ‘important’, that important thing never having being specified. Noir assumed it was some sort of threat, possibly to do with The Boys. Maybe Homelander wanted you with someone he trusted. In that case, Noir could understand.