I doubt that childhood vaccines are why the kiddos aren’t getting sick. Their immune systems are just different. It’s why they get chickenpox and mumps and it’s not a big deal. Get those at 30 and it’s a really big deal sometimes.
Doctors and nurses particularly get the ‘whole schedule’ before they start rounds from what I understand. My niece who is a pharmacist did too. She got like 5 or 6 jabs over a week or two.
Probably due to the size of a child’s body. Children have to be born with an immune system no better than their mothers. It may be just the ratio of immune capacity to body mass. And then of course biased by organs that may not operate as efficiently as when their were younger. IMHO.