I believe it is not possible to make a statement that applies to all taxa of silver dollars. Also, it depends a lot on what/how the silver dollars are being fed, and the characteristics of the aquarium situation.
The term "silver dollar" encompasses a vast array of taxa in several genera and dozens of species. They occur in a variety of habitats, from fast moving, rheophilic conditions, to slower moving bodies of water. Not surprisingly, they also include a variety of feeding strategies and adaptations, with some species being more toward herbivory, and some more toward invertebrate animals, with most being somewhere in between, along a continuum, being highly opportunistic, and (for many species) being affected by dramatic seasonal changes in their habitats (high, versus low water seasons, phenology of riparian and aquatic plants), and thus diversity of food items available. Thus, it is not possible to make a statement that applies to all taxa.
In my experience with several species of Metynnis (I keep 4 spp.), they generally will not eat even the smallest fry of various species of co-habitants. When breeding guppies, platies, and rainbows (in other tanks), I often purposely put the fry in the silver dollar tanks, because they would survive there with virtually no loses, whereas their chances of surviving in the guppy or rainbow tanks were much lower or nil.
For the species of Metynnis I have kept, my experience is that they will NOT actively hunt small fry if present in their tank. Whether they may, on occasion, eat some fry 'accidentally' I guess it is possible, but I have not observed even that.