Age doesn't really determine whether Xenforo is right for your project. The age factor applies more to the nature of your site's content rather than the platform you use.
Children's thumbs have evolved immeasurably since my simple up up down down nintendo days, and I see primary school kids with a better phone than I have. With relevant content, I see no reason why not.
Given that the age range will mainly be using smartphones and tablets, Xenforo is definitely the best forum software with its responsive design. Laptops and desktops don't exist in their lives outside of school...