I need help with motorized projects.

I want my kids to play with closed circuits and switches. I want them to use motors and accessories ( wheels, gears, fans, etc.) however we keep having attachment issues. I have a few motors but no easy way to attach the wheels and gears. My wheels and gears are from various closed play toys. It isn’t easy to affix them to the motor shaft without slipping. I searched unsuccessfully online for a hobby shop that sells small dc motors with an array of easy to attach accessories without having to use universal mounting hubs. I feel like I’m not speaking their language because I didn’t see any obvious solutions. I want something easy so my kids do all the work themselves with my guidance. I don’t like linear kits like Littlebits because it doesn’t teach closed circuits. I want my kids to use wires and batteries. I’m just having issues so far with the attachement of motor shafts to wheels or gears. I’m sure someone must make what I want. Does anyone have any ideas? Also affordable would be helpful?

Thanks, Lori