What do you call a car that doesn't work? A car-wreck! (said with a playful tone)
Why did the scarecrow win an award? Because he was outstanding in his field! (A bit of a stretch to cars, but connects to driving on fields, and kids love scarecrows!)
What kind of car does an egg drive? A Yolkswagen!
Why did the bicycle fall over? Because it was two tired! (Again, a bit of a stretch, but the pun works and it's silly)
Remember to tell these with enthusiasm and silly voices to make them funnier for kids!