Per Muffin, the rules for playing these classic games are:
HIDE & SEEK: Muffin hides, and counts to 10 while hiding. When she's done counting, the other person says "I'm coming to find you", at which point Muffin darts out of her hiding spot.
TAG: The person who is "it" runs, and the other person tries to catch them. When you catch them you become "it" and have to run away and "avoid" being caught.
I know - she's got them backwards! Its so cute though ;) We've tried explaining to her the 'typical' rules but she doesn't buy them. Gotta admire how she likes to do it her own way!
On a related note, I think this NPR article does a great job explaining why creative play is SO important.