Hi Lauz!
Very cool to hear their interest! I've got two suggestions- one is more formal, fhe youth apprentice program at Bikes Not Bombs in JP (I'm a board member there for full disclosure) which gets youths introduced to bike mechanics, working in the shop, advocacy work, etc. BNB also has an "earn a bike" program.
The other is less formal, which is the Somerville Bike Kitchen, which is between Porter and Union Square. Its an all-volunteer bike repair space, where people can come in to learn how to repair their own bikes in a hands-on way. I'm not sure how much they've worked with youths, but I do know a few of the main volunteers there so would be happy to connect you.
James