As the only GP to 4 GCs and my SiLs work schedule I would be invited to the Nativity plays and birthday parties while they were preschool, and infant school. Once they moved to junior school, around 7 years old, not so often.
As to birthday parties, if it's large hall parties, then I'm included, but small themed parties, then no.
For the youngest, who will be 7 on the 27/12, I usually attend as her party is usually before school breaks up then I see her and everyone on the 27th for a panto visit, a tradition since the eldest was 5, (now 16!).
I don't see them over the 25/26 for various reasons, all amicable and through choice, discussion etc but visit before for present giving and after fir panto, and I receive oodles of photos, texts and videos on the 25th. A win/win for all of us.
For the OP, maybe ask your son/DIL if you could see the Nativity, if possible, and discuss doing something for their birthdays when they visit for tea and cake, maybe take each child out for the day close to their birthday, just you and the child. That way it's a special time for all of you.