It's always tough to come up with something witty to write in a birthday card, especially if it's for a kids birthday. Half the time they don't even read the card, either because they don't know how to read or because all they're interested in is the gift. Even still, it's important to a lot of people, including yourself, to come up with a funny birthday card message that the kid will actually find amusing.
For tots, you can make fun of the fact that the kid isn't even old enough to be able to read the card — it'll surely give the adults a good laugh, and let's be honest, they're the ones that really care what the card says anyway. And for older kids, sarcasm and puns can be home runs. Just make sure that you keep it short and sweet, because kids have priorities and that priority is to get to whatever is under that wrapped box.
If you're really creative, you can also make the card interactive. Draw a cool illustration for them to color in, sketch a simple maze (or print one out and paste it in) for them to do, or turn the card into a map for a scavenger hunt that they need to complete to find their gift.
Either way, you can't go wrong with these clever messages to write in a kid's birthday card.