Before your grand kids decided to stick with this game, like Typhoon said, a couple of the U.S.Kids club will do.
I would even look at your local Good Will sports section. They have website online so you could save the trip.
I see almost brand new kids golf clubs all the time at my local store.
If you have a local First Tee program, they should supply the beginners with golf clubs.
I know everyone just want to adore their grand kids. I would make sure this is something they like to do before spending money to show your love for them. You will have plenty of time to show your support should your grand kids decided this game is for them.
I started my kids with just one U.S. kids 6/7 iron. I believe I still have it somewhere because my youngest one forbid me to sell or to donate it, even after they had grown out of it long time ago.