Yeah, prunes are dried plums. (In fact, the California prune producers started making a big push a few years ago to call them "dried plums" instead of "prunes" because some people have negative associtions with "prunes.") I think that the baby food flavor you are thinking of is basically pureed stewed prunes (not pureed cooked fresh plums). The prunes would sort of plump back up when they're cooked in water, but the effect would definitely be different than just cooking plums. I think that most packages of prunes (er, dried plums -- sorry, California!) have a basic recipe for stewed prunes on the back, which you could adapt as needed.
ETA: For recipes for both plum puree and prune puree..