This is actually one of the main reasons I had for coming out as aro/ace. Not necessarily only grandchildren, but also remarks about getting a girlfriend etc. Coming out had the intended effect though, it has effectively stopped these remarks from occurring.
Meanwhile I unfortunately don't really get the option to come out as being Ace, or Aro, to my parents, more so my dad due to his wild beliefs about the world from conspiracy theory stuff he listens to every day. And my dad is the one always making remarks about getting a girlfriend, and though the grandchildren part isn't included, it's implied by his expectations.
Yeah that sucks. I've realized that I'm overall quite privileged with my surroundings. I had expected some people to be annoying about it, but it all went smoothly. Sadly that's not the normal way that coming out as ace or aro will go, mos people will be met with some people who don't understand it or who actively hate it.
I try to be quite outwardly aro/ace and queer in order to take the bullet for others (as much as I can handle ofc).