Grumpy Grandpa - AIBU?

Having the GCs during school holidays can
be exhausting no matter what their ages. Your husband may not have the stamina that you possess so perhaps give him a rest sometimes whilst you take the kids out and ask the 16 year old to help out with his/her siblings, is certainly old enough to do this.
Cannot imagine is enjoyable for you either to watch your husband reaching the end of his tether, you must be on pins, so would be more enjoyable to let him have a day off every so often.
Obvs snapping at the 11 year old wasn’t good but it depends on the context, what was the 11 year old doing at the time. You say they are excitable and noisy but they aren’t small children so this should be more manageable, if it isn’t then do have a word with them, they are old enough to be negotiated with.
All the best with this.

Has he always been grumpy and easily irritated or is this a relatively new trait in his behaviour OP?
The children are of an age to know how to behave in a restaurant surely but had the 11 year old overstepped a boundary on that occasion?
You aren’t being unreasonable to feel upset with him but there might be an underlying cause for his short temper and low tolerance. Even a UTI can present as such so maybe ask if he feels unwell or does he have any other symptoms.
Consider monitoring him if he won’t open up and discuss this with you and depending on what you notice, think about making an appointment with his GP.
Enjoy your time with the DGC though, they grow up much too fast!

Well if it’s not his thing (and not everybody enjoys time with young teens) let him stay home, and you do things with the teens on your own. I doubt the 16 year old will wanting to be ‘hanging out’ with grandparents much longer mine were all off doing their own thing with their friends at the age and younger

Let grandad be free of these duties and chose to come if and when he feels he wants to, after all they’re not toddlers to be looked after enjoy your time with them and don’t be worried about the fourth teenager