Final Idiom Idea

Don't put all your eggs in one basket: Birds will be betting on an earthworm race in the front yard of some suburbian house. One bird slaps down all his money on the fastest, best earthworm who typically wins all of the races he enters. Another bird advises him against it but bird #1 is very sure of his decision. They settle down for the race start and the gun fires. As soon as all the earthworms take off a kid runs through their "track" and squishes the fast worm. Animation ends with a shot of the birds' mouths hanging open.

Other Ending Ideas: Another bird or animal swoops in and steals the worm. This will call for extra animation and modeling as opposed to the foot/shoe of a kid.

Idiom Ideas

Kick the Bucket - Could be fun
Like a Chicken with its head cut off
You are what you eat
Back Seat Driver
Break a Leg

Don't judge a book by its cover - old bic pen replaced by those fancy multi-ink pens. All the kids are using the multi-ink pens and they laugh and make fun of the plain pen. In the end the plain pen transforms into some mega-super-ultra pen and totally owns the fancy pens.

Jaywalk - Done in the Fashion of the opening sequence for Saturday Night Fever. Towards the end the camera snaps to two birds watching from afar where they deliver the punchline (the bird walking is a bluejay).

http://www.tcm.com/mediaroom/video/294764/Saturday-Night-Fever-Movie-Clip-Open-Stayin-Alive.html