Bugs attacking my baby rhubarb seedlings

I'm surprised that anything is bothering rhubarb. First, I'd address just where you have the seedlings placed. Is it too moist? too shady? too close to something greenfly normally loves?  Obviously there's something close by which the greenfly loves so I would move the seedlings to another area after you get rid of them.

At this young stage, I'd just use a regular soap and water treatment on both sides of leaves just to get rid of the pests, and gently scrape off any egg clusters you can see. Once you think you've removed all pest signs, then move to a new location which is hopefully free of the usual attractants for greenfly. Hoverflies love them so perhaps a spot where the former seem to gather.

No, not midges.  Hoverflies are larger and look like bees or wasps (just looked for pictures with google images and there are apparently lots of different sorts that look a bit like different species of bee - I never realised!) but don't sting.  You can see them quite clearly hovering still in the air, then darting about.