When creating a volunteer role aimed at young people, don’t use this word in the title. Screw organizational hierarchy – if your title is “coordinator”, don’t just focus on role names that sound “beneath” you.
Using “assistant” trivializes the role and can create a mindset that this person is someone that be sent all the administrative crap work to, rather than someone who is a valuable contributor. (While we’re at it, if that’s what you’re actually looking for – someone to send all the administrative crap work to – you should probably be rethinking that too).
It also is not as impressive on a young person’s resume. If you’re looking for a volunteer with a bit of experience, use a title that helps them build a narrative of roles with increasing responsibility.
Here are other words to try out: