The key to effective mentorship is 1) finding someone who has something that you want 2) helping them, for free, get something they want. As a result, you gain access to the mindset that has the thing you want.
Perhaps you could ask the kids who they really admire, or what products or things from their own lives are impacting them. I'll bet there are unknown engineers and scientists behind just about every single man made thing they come into contact with on a daily basis.
Reach out to those companies and see if they will put you in touch. Try the PR department possibly for the intro. This will give the kids a real connection to the people.
If these are products where the kid is the end user, the engineers might love to spend an hour a month on skype with them just to gain insight into their lives to make better products.
Some people might agree to help kids because they want to, and that is great, but you will 10x the value for BOTH the kid and the scientist if you find a way for them to really help each other (and it's massively possible).
I've mentored high school / college kids into pro athletes, wildly successful entrepreneurs, activists and non-profit leaders and it's always been rewarding.
Finally, the process of finding a mentor is a skill that most people lack -- what a great chance for you to involve them in finding their own mentors and letting them reach out, send emails, and otherwise discover the power of their own voice and agency.
Fun Fact: When I was in 5th grade back in the late 80s I had a secret pen pal / mentor I found in the back of 2600 Magazine who was in federal prison for hacking ;)