Thanks for getting in touch. This sounds like a great workshop for young people! If I understand correctly you are interested in using the Bela board as a kind of "black box" synth where the students won't have to worry about editing code or anything like that? They will just be able to make sound once they assemble the instrument.
Here is an example of a simple synth which runs with the Trill Square: https://patchstorage.com/trill-square-synth/
It was created using Pure Data which is a bit easier for beginner users than c++.
We don't currently have a default example which uses both Trill Square and Trill Bar together but one would be easy enough to create. Do you have an idea of the kind of thing you would like to control? One could control the synth and another an effect for example.
From my experience running workshops with young people in the past, sampler-based instruments are a big hit as then it is very easy for the students to customise the sound of the instrument by changing the samples. There are some examples of this kind of thing on Bela with Pure Data – have a look at the sample-scrub example.