We had a party for our DS and DD (who are 1 and 4 over xmas) and we didn't invite grandparents (from either side) Just cousins and some friends of DD's from playgroup. Yesterday my MIL came in and DD said to her "It's my birthday today!" MIL looked all superior and said "Oh noo sweetheart, it's NEXT week"
DD told her it was today cos she got lots of presents from her friends and cousins.. we then got the 3rd degree as to why we had a party and never told them!! (neither sets of GP's see our children often and have no day to day involvement in their lives) My mum was okay about it, she said she couldn't have stuck 20 screaming kids all afternoon, and I dunno why MIL was so annoyed anyway, she had a hair appointment!
Let them stew in their own juices and don't let them make you feel guilty.. it was for the kids, let them go and grow up!! Thats what we've decided to do with ours!!