Buy MP3, download, burn CD, play in car. Kids can choose any CD that's in the car or regular radio.

Download MP3

Put on Plex/Jellyfin/Subsonic

Play in car

My kid is only 2 years old, but when he can understand the concept I will be sure to teach him that he doesn't have an arbitrarily large choice, he can only choose from within the options we have available.