I've had good use out of the PilotUSA pink one on the page Travis linked when my daughter was 5. That model has an input for 1/4" audio wire from MP3, DVD, GameBoy, etc. She likes to chatter to herself, so I've pulled her mic wire on more than one flight to avoid her breaking squelch and pouring cabin noise into my head non-stop (the avoidance of which is why I wear headsets in the first place). She can hear me; I ask her yes/no questions or just read her lips.
For my son, we used the Peltor Kids passive muffs from age 2-3 with good effect.
The key thing is to try them on for fit and make sure you get a good seal on the cups above their jawline.