Do an iron on or write on the onsie with a sharpy.
I got my mom a "grandmother favorite" onesie for mother days. Also got myself a tank thay say "due in December".
Can't you get a 'Grandparents' Christmas card and pop a scan pic in there?
You could give them each a little box with a set of booties in it with a card that says something about being a grandparent. That way it's a keepsake for them.
Ooh I love the booties idea!
Etsy! They have tons of custom items people can make for you