If you have the inclination and the funds to make a down payment on your son and daughter-in-law's first home, do it. But don't break the bank and set yourself up for financial ruin.
It's a once-in-a-lifetime event, sure, but "lifetime" is the operative word here, so make sure you keep enough in the bank to comfortably live out the rest of yours. Your son and his new wife have decades to build and acquire their assets, and while it's true they could probably use the help, they'll be OK without it. You managed to get by without a massive donation from your parents, and they will, too.