I have SUCH a hard time getting my kids to just sit while they eat. They are constantly getting up and down off their chair and getting water from the fridge, or picking up something they dropped, or getting a toy (even though toys are not allowed at the table...) The kids know I know they know I want them to stay in their seats until they're done. There is always some kind of excuse. They area learning that excuses involving things I love, may give them a better chance of getting away with things:
Me: Peach, are you all done eating? Is that why you're not on your chair?
Peach: Oh, I just needed to do my ballery. (She quickly puts her hands over her head in some kind of mock ballet-pose.)