Christian's birthday party was a success. Thanks for everyone's suggestions.
We made a fabulous race track cake . . .

Painted and assembled pinewood derby-style cars . . .

Played "Pin the Wheels on Lightning McQueen"
and whacked a pinata . . .
Had to keep the activities indoors due to the still swamped backyard.

Happy Birthday Dear Christian, Happy Birthday to You!
Ice cream, cake, presents, playing and fun!

They decorated goody bags (white paper lunch sacks) to look like traffic lights. We filled them with a Hot Wheels car, Hot Wheels Fruit Snacks their painted car, a Learner's Permit, and the candy. The two girls also got a little Kelly doll.
The thank-you notes turned out cute, too. They match the invitation, and I created driver's licenses with their photos on them. I laminated them and put them in the note. Cute!