They have two really great pools. They can play in the sand at the beach. You can rent a mouse boat and take them around the lake. Play video games in the Arcade. Go for a monorail ride. Take them on the boat that goes to the MK.
There are also organized arts and crafts plus games at the pool in the afternoon. For ten and under there is the Wonderland Tea Party and Grand Adventures in Cooking.
There are lots of things to do in a Magic Kingdom Resort (such as the GF). You can take them on a Guided Fishing Excursion. You can take them to a Golf Outing (there are even "lessons" that may be taken). You can let them Play Tennis (there are also "lessons" that may be taken). Water Sports are also something that they may enjoy with you. According to the WDW Website Regarding the Grand Floridian Resort, there are swimming pools to enjoy, white sand beaches to enjoy, and so on. And I can't end this post without mentioning the incredible Children's Programs that are offered for children of a variety of ages. Every child who has ever vacationed with me at the GF has had a magical time. *Sigh* Enjoy yourself!