That said, my guess would be normal reinfections would be less severe, more like a cold. There have been some severe reinfections but any reinfection under six months suggests something atypical in the immune response.
Also SARS-Cov-1 produced a longer immune response so that may happen here.
But lifelong sterilizing immunity seems pretty unlikely based on what we know so far. The vaccines aren’t aiming at sterilizing immunity actually, their goal is to greatly lower the severity of the infection.
You can get limited immunity from the common cold after being infected but you are just as likely to catch it 12 months later anyway. With covid that means we are dealing with it annually, which our health services cannot keep up with.
"Common cold" and "flu" is not 1 virus, but hundreds of different viruses which is why you get them repeatedly. You do build immunity though. It's why children always have a cold and adults a lot less common.
If you can't get immunity from the infection, how will the vaccine work?
There is a difference between the level of illness experienced by those who are vaccinated Vs those who get infected, so more old people will die as a result of infection than would die as a result of vaccination.
So no, it would not be better for healthy adults and kids to get it in Q2 2020. Also how do you propose that you only infect the healthy adults and kids?