Okay I'm not really sure what you're trying to do and where you need the interfacing at, but I'm wondering if headliner cloth might suit your purposes. It's soft and flexible, but will stand up straight. I use it to line handbags. Joann's, Hancock's, and (I think) Michael's carries it, and I would imagine Hobby Lobby also. I'm not sure about HL because there isn't one in my area.
But check it out, see if it might suit your purposes.
p.s. Are the kids in Texas? I'm thinking flannel might be really, really warm....? Of course, headliner fabric might be warm, too....