I was 11 when I took my 1st sewing class -- I still remember the beautiful multi-colored windsock from summer classes.
I would definitely pick a log-cabin or other similar straight line design for the doll quilts.
For the 9 year old, it might be a good idea to have some Fabric Fusion glue on hand so that she can still experience something if the sewing proves to be too much for her. Some kids really love picking colors/layout more than the actual sewing, some love machine stitching & others love a simple hand applique like a single heart in the center. If you can, maybe bring a couple cheater needles along in the event that is something that is more interesting to her.
Also, unless they really love sewing, you'll get about 20-25 minutes of attention from the 9 year old & about 35 minutes or so for the 11 year old. So one thing you might consider is whether to seam the edges (or serge them if you have a serger) versus applying binding. I've worked with 8 year olds on sewing projects & honestly some of them absolutely LOVE cutting. If you have a Klutz glove, it should be fine to let them use a Rotary cutter after a stern warning that if they are quick or careless they could end up having to get a bunch of stitches at the hospital (hey, it's happened to experienced adults!). With kids I've known really well, I've even let them do some cutting without a Klutz glove but that requires some judgement as to how exactly they can follow instructions & curl their fingers/keep them away from the blade. I always make sure to put on a new blade & get my scissors sharpened just before I have kids use them because the sharper they are, the less likely they are to slip or catch and end up off-track.