Because they are much less likely to become seriously ill, need a hospital admission, need ICU or die from Covid-19. Yes, it may stop them passing the virus onto people in over 60s or the risk groups, but the effect would take a long time to work through the population. Also with all the parents who are anti-vaccine (think lack of MMR compliance), there is a strong possibility that many children would not be vaccinated.
Having been involved with primary care, the level of resistance to vaccinations by parents is quite worrying. There tends to be less resistance from the older population, many of whom remember the mass childhood vaccination campaigns of the 50s & 60s that effectively minimised / removed diseases such a whooping cough, polio, diphtheria etc. that had previously been killers.