This is such a small thing - but honestly. No present, no card, not even a phone call.
They have a lot of grandchildren and see lots of some (mainly the ones who live round the corner) and almost none of others (we live many hundreds of miles away but visit every few months; there are GC 20 miles away they see less). So fairly hands-off.
MIL will complain about kids never phoning her but won't call because 'she doesn't want to butt in', to give you the general idea.
But I know the see-lots-of GC get birthday presents, so it isn't a policy. They've just forgotten.
Feel can never say anything, even to DP. Don't know if he's embarrassed or hurt or what but don't want to make a big deal of it just in case.