I took a Quilting 101 course that had double irish chain as the quilt. There was a lot of corners to butt, however, I got real good at it. Let's face it, butting seams is important in quilting. I can see why some would recommend rail fence though.
Do you plan on having each one make their own individual quilts? If you taught them by all three of you making one quilt (double irish chain), they each would have different homework (if you will) and the quilt could be a traveling quilt. One that holds many memories of all three of you working together....just a thought. Yes, I'm kind of stuck on that one quilt as a good first quilt to learn.