Good question with youth depending on there skill to be able to maintain harmony my suggestion would be 2 things. Sing hi energy songs that are unison. That way they all can focus and learn about singing the melody while still enjoying themselves if you'd like i can put some examples of songs up.
My 2nd suggestion would be have 1 or 2 Older adults singing in their respective sections to help them maintain there parts as they sing. Working with youth I would suggest the adults love kids and are fun.