I didn't take the Peds GI boards, but I did take the Peds ICU boards and used the AAP PREP ICU questions and thought they helped. Additionally, you can claim CME toward MOC if you do them.
For PICU, the Society of Critical Care Medicine has board prep materials. Is there a national GI organization that has similar resources? Additionally, ask the GI colleagues you work with, I bet they have all sorts of materials and resources.