I smiled immediately when the idea sink into my brain. Fun and good thought!
Some real life references would've helped you to get their anatomy more accurate I think. Grandpa's other thigh is thick while other one is slimmer. Daddy's bottom part is too small compared to the upper part even if he's in the movement. The kiddo looks pretty good.
Some extra attention to the background perhaps..? Especially in front to create even clearer perspective. It might not be nothing but few more sharply drawn grass leaves. :)
Character design you nailed in my opinion, though! Their clothes look great and tell the story for the character carrying them. Well done!
Good luck to the contest! :)