Pimoroni have a great guide to installing and configuring software for your Pirate Radio Kit here.
Although a sample playlist is provided, here's some links to get you started looking for some kid-friendly/educational streams:
- 20 podcasts for kids - Get started with some kid-friendly podcasts for primary-aged kids
- 12 science podcasts for teachers and students - This is a great list of science podcasts for teachers and students from ages 8 onwards
- The MakingIt podcast - For makers, this podcast is full of interesting projects involving robots, tools, and interesting materials
As mentioned in the Pimoroni tutorial, to add streams to your /home/pi/.config/vlc/playlist.m3u file, look for links to RSS feeds on the sites above, and copy these into the playlist file.