While not truly a Keyguard this item will still Guard against little fingers using the iPad Home button to leave the app they need to be working in to "do something fun" instead.
This "guard" has a center opening of approximately 7 3/4th inches by 5 11/16th inches to allow full access to the app in use. Can be used in either Portrait or Landscape orientation. Can be held in place with Velcro if needed.
iPad Home Button Guard