I read somewhere recently (because I have a one year old and was concerned) that younger children's nasal cavity isn't quite mature/formed enough for the virus to incubate. The launchpad for Covid is way up in the nasal cavity.

This is also probably why most tests have to have a swab feel like it is touching your brain through your nose in order to get a sample. Hell, I wouldn't be surprised if these tests are suspicious now that I think about it.