OR immune damage due to covid itself, which was a more popular theory with more mainstream medical support last time I looked into it about ten months ago.
This isn’t true (that your hypothesis is mainstream), there is no evidence the virus itself was more severe or resulted in more severe illnesses than expected to support the impaired immune system theory[0].
There is endless literature on this but the majority of “mainstream medicine” subscribes to the “every kid got sick at once for the first time” theory[0-2].
Anecdotally, I was working at a pediatric hospital for parts of the RSV wave and my conversations with PICU/ER colleagues was condordant with literature in this space.
https://onlinelibrary.wiley.com/doi/abs/10.1002/ppul.25582