Personally, bluegill are not a bad choice of fish to grow.
However, I don't have experience with how fast bluegill go from being fry to being fingerlings. I do know that with very warm water (86 F) and very high protein (50%) meal fish food the tilapia fingerlings will grow very fast and could be nearing 3 or more inches in as many months.
for about a week after they are let out of the mother's mouth the tilapia fry resemble swimming eyeballs. Or as I called them eyeballs with stomachs.
Bluegill lay eggs and guard a nest while tilapia are mouth brooders. I don't know if the bluegill protect the fry after they are hatched so your description of orphaned makes me think tilapia but were you really there to see the otter attack to know the tiny fry were under the protection of a parent to be able to call them orphaned?
I do know that sunfish ( related to blue gill,, protect the nest like all bass and cichlids) my dog discovered that his paw in a nest would intice the sunfish to bite his paw the would then toss the poor fish up on the back to eat,,, I miss my lab :)
I would load a pic but it does not seem to work will try again,,, or perhaps I could send the pic to other members and have it posted?
To upload photos you may need to reduce the size a bit. Pictures of about 600x800 or less seem to work pretty good on the site for viewing. jpg usually uploads fine.
Or when I first joined the site I had all sorts of problems with my firewall and security software, I had to switch to something that would let me deem the site safe so that many of the functions of the site would work.