If they have 3 kids at private primary school, that is a nice bill they have there. I'd say $20k a kid, so $60k a year.
My guess is DC earns between $400 & $500k per year, so $230 - $290k after tax.
$60k private fees would take up around a quarter of his income. That's a fair whack.
It's possible that they could be financially stretched but obviously this would depend on their mortgage level and spending habits.
This is tacky and pure speculation - but my bet is they are not financially independent and have some family money. I would be surprised if their properties, school fees, and lifestyle was paid for 100% with his income.
They would need to be saving for the future, and doing things like travelling to Boston to run in a marathon is very expensive. It's not really something a one income family with three kids in private school would do.