Ask HN: I need to organize a hackathon

I was asked to organize a hackathon for schoolchildren. This hackathon will be held for three days. All children are high school students and winners of Olympiads in Informatics.

I am creating educational program consisting of mobile app development and web app development. I need your advice on selecting actual tools for both parts. We can assume that children has no prior programming experience except of programming in Pascal at school.

For mobile app development I want to use MIT App Inventor. I think that it is right tool for quickly prototype their mobile app. Learning Java and Android SDK for three days is unrealistic.

And for web app development I have no idea. Rails? Django? Flask? NodeJS? First I need to teach them Ruby, Python or JS. Apart from that I have to teach them HTML/CSS/JS and basic concepts of web.