You're still missing the point, Allan.
1.) Children with the virus are vectors. They spread the disease to others who can suffer severe disease and death. Remember Typhoid Mary? She unknowingly infected 53 people. Three of them died. She, herself, was asymptomatic.
2.) Children can appear to be unaffected, only to develop MIS-C weeks later. If left untreated, it can be life threatening. No one wants to knowingly subject their kids to that possibility.
3.) The risks you can face from getting the disease are greater than the risks from the vaccine. Pericarditis is a good example. Your chance of developing pericarditis from the disease, itself, is several times higher than from the vaccine.
All the vaccine does is teach the body's immune system what the virus looks like so when it sees it, it can launch an immune response. Your immune system does the heavy lifting, not the vaccine. You are correct when you say we don't know how long your immune system will fight off infection. Not enough time has passed to really know yet.
Immunity is multifaceted. When your body senses a foreign substance (virus), it begins to produce antibodies. Our bodies encounter foreign invaders all the time so they are constantly producing some form or other of antibodies. They are too numerous for your body to retain on a long term basis. That's why efficacy wanes. There is good news, though. Once your immune system knows what a particular virus looks like, it starts producing special cells (T-Cells and B-Cells) that are specifically tasked with sensing the presence of invaders and subsequently making a new batch of antibodies. People who were infected with SARS-1 in 2003 have long since lost any antibodies they developed. Recent tests, however, have revealed they still have at least some T and B cells, 18 years after the fact. SARS-1 is similar to the current SARS-2 that is causing Covid. With that in mind, it's reasonable to assume that some T and B cells for SARS-2 may last for years, as well. We don't know that for a fact, though, because not enough time has passed. In the meantime we can use the vaccine to tell the body to make T and B cells and keep the arsenal full just in case .