Your scenario sounds plausible as far as the babysitter angle. And that may well be what the initial trip was meant to be. I do not know for certain if that is what happened, but it sounds logical.
I think, however, that this was truly a double abduction, rather than a single abduction and run-away combined. If Theresa (age 12) had been abducted, or simply walked away, I think that Cynthia (age 16) would indeed have gotten upset. But I think the more logical and normal course of action would have been for her to seek help in locating the child, or in reporting the abduction (if she witnessed one). Even if she did get upset and run away, I think that she would have gone home or somewhere very soon afterward.
A pervert wishing to abduct one girl could have used the other girl as part of his scheme. For instance, taking the younger one and threatening the older one to get in his vehicle or he would hurt the little one. Or he may have simply decided to take both of them.
A double abduction is somewhat rare when compared to all known abductions. That is one thing that makes this case a puzzling one. See also the featured topic of the Lyon Sisters, missing from Wheaton, MD or the threads on the other cases in Cold Cases. Could there be other common links besides the fact that they are double abductions?
With the double abduction having gone unwitnessed in most or all of the above mentioned cases, one is left to wonder just how he did it. What would make two girls enter a stranger's vehicle. Why had nobody seen them get into a vehicle? Was force used, or were they enticed in some way?