If there is one thing I've learned from being a nanny, it's that children photograph best when you give them absolutely no direction for the photo shoot. Don't have specific expectations in regards to specific shots. Just dump out all of their outside toys, and let them do their thing. Let them use their cheezy, fake smiles, let them run around and hang upside down. Let them cover their face and pretend to be monsters. I promise, you won't be disappointed.
Perhaps I am a bit bias, but I truly believe these two are the poster kids of childhood:)