3 Hours is not much time to work with one person on a quilt, let alone 2. But, I'd say just sew strips together till you get to the size you want and then sew the quilt together using the birthing method. You won't have time for stitching on paper, etc. Personally, I think the key to winning your nieces over to quilting is what fabric you use. Pick something they will fall head over heels about and you'll have them hooked. Kids are visual..... and fickle. They'll either love it or hate it.
Do let us know how it goes.... we all have younger nieces, nephews, and grandkids that might someday want our help to learn how to sew.