Needed for context, when talking about kids and immunology:

It is physically impossible to train the parts of the adaptive immune system that haven't developed yet.

They have to be there to activate.

This is not a fringe claim, vaccines are designed with this in mind:

The transient delay in IgG2 antibody production is clinically relevant in the context of vaccination, as this subclass is mainly responsible for the response to polysaccharide antigens.This fact explains why response to polysaccharide vaccines is impaired in children under 2 years https://pmc.ncbi.nlm.nih.gov/articles/PMC6817213/

So if someone tells you an infant under 2 needs to be exposed to pathogens, without being able to tell you which pathogens have polysaccharide envelopes and what that means in the context of infants acquiring immunity, be on guard

And some pathogens are harmful even if adaptive immunity is activated!

If that infant catches RSV (or EBV or SARS2 or influenza or multitudes of other airborne pathogens) there is a serious expected cost and no known advantage.

I am not an immunologist, if I've misunderstood please correct me.