Sounds like they have a lot going on already. I'm the type of parent who think kids need some unstructured time to be creative.
What about programs at your local library? My local library always has summer programs.
How about suggesting that each kid film their own movie, write and direct. They could get friends to act in it, claymation, or animation. After filming they could learn how to edit it on the computer. Could take up a lot of fun creative time. At around this age my son got a claymation kit for Xmas one year he had fun with.. but it can be done by just buying the right kind of clay at a art supply store & the camera on a cell phone.
What about a Rasberry Pi?
https://www.raspberrypi.org/ There are a lot of websites that have projects for kids. This might be easier if mom or dad or someone you know can help when things go wrong.
Probably above your budget but If your willing to spend a lot of $$ on supplies. You can buy your own Lego robotics Mindstorms Kit. There are instructions that come with it and many different Robots can be made from it. Most kids I know learn how to do this at Robotic's camps & clubs these days but it's not necessary.
