I've noticed that when I use the "Elixer of Life" that the order of my Sims changes. So yes, the listing is by age. The oldest members on top and youngest on the bottom. So if you wanted sim 2 to be below sim 3, you could give sim 2 enough Elixer so that he/she is younger than sim 3. I'm not positive if this works on children because I've never done the Elixer on them, but I've done it with Teens through Elders. Unfortunately, this won't work straight out of CAS because you won't have the aspiration points needed to purchase the Elixer. Hmm...maybe a buyable Elixer?
Also, have you tried manipulating the "days left" in the life cycle while in SimPe? I would think that would determine how old they are and maybe force the thumbs the way you need them. I'm not positive tho... But if you back up your hood, which you should do anyway before messing with SimPe, you could always restore the family if it doesn't work properly.
And don't give up just yet cause this message is still new and more people, with more experience with doing this, may just chime in.