There is an unwritten rule in the Halloween films that Michael Myers does not kill children. There is no real explanation for this, as we've seen Michael dispatch teenagers, adults, and even animals without prejudice. It seems particularly odd Michael would draw the line at children when his whole purpose in life is killing. We do, however, see numerous instances when he chooses not to kill children. For example, Michael spares both Tommy and Lindsey when he could have easily killed them in the original film. In Halloween (2018), Michael encounters a baby crying in their crib, and he leaves them alive even though killing them before their crying could draw unwanted attention would have been in his best interest.
The reason Michael seemingly won't commit violence against children has been debated for years. Some fans believe it's because Michael doesn't see children as a threat. Others think he still sees himself as a child, so he identifies with other children. At the end of the day, though, we don't know why Michael doesn't kill children. It has never been explicitly elaborated on in canon.
It's also worth noting many fans aren't convinced about this unspoken rule, arguing Michael has killed at least one child. Many cite the killing of a father and son who encounter the prison bus Michael's being transported on in Halloween (2018) as an act of violence against a child. The son grabs a gun in hopes of defending himself, and Michael kills him quickly. The problem is we don't know how old the kid is. Many believe he looks young enough to be a child, while others argue he's in his teen years. Either way, he took up a weapon against Michael, making him a threat. Perhaps, in Michael's eyes, he was no longer a child because he could hold his own and fight back?