I have made shirts for my mom before. One year I got all of us to do handprints on it with puff paints and wrote the name under the print.
One year my SIL got my dad a sweat shirt with a picture of my niece (she was the only one at the time) on it, with matching sweatpants and wrote her name real big down one leg of the pants.
More recently someone got my dad a shirt that says Grandpa rocks and it has a picture of an old rocking chair on it.

Ideas for sayings:
Happiness is being a Grandma (which she probably already has, it's pretty common)
Grandma's pride and joy
I love being a Grandma
My seven (7) wonderful grandchildren
7 little blessings