Sure, childhood immunization is great but we're not discussing measles here. Measles is a single, relatively stable virus that's easy to vaccinate against. But children in daycare facilities mostly get infected with upper respiratory viruses. There are literally hundreds of such endemic viruses, especially rhinoviruses but also coronaviruses and others. We aren't likely to have effective vaccines against those any time soon. If children aren't exposed to them in daycare then they'll be exposed somewhere else. Those viruses aren't going away and will continue circulating through the population no matter what we do.