First grade teacher here. One week, totally fine. Two... eeekkk.... not going to be a deal breaker, but honestly, it’s not the norm. Luckily, they are pretty far apart, so that helps a lot.
Are you planning any other vacation days? A day here? Two days there? Is your son a sickie? I have two kids and between them, they’ve missed 2 sick days in the last three school years. Total. So, I felt pretty good about them missing two days for
Disneyland last year (which were the only two days my kindergartener missed all year... and yes, i had Dumb guilt about screwing up her perfect attendance. My son broke his arm so he had missed a day earlier.) But some kids get every bug and it keeps them out for 3 days each time. I’d hesitate a lot if I had a kid who is illness prone.
Its fairly insulting to say it’s “just first grade” but it is much easier to absorb absences when they are younger. If this is the only chance for an extended family vacation, I’d take it now and have a great time. It’s ultimately your decision and as long as you don’t miss more than a couple other days... it shouldn’t be too impactful.