I imagine their bums were a little red and swollen from the poo and from the cleaning. I would be cautious of putting something like preparation H on chicks so young. I have not heard of doing this so far. Maybe someone else could chime in on that aspect. I know that it has been applied to older hens for prolapses but never use a medication that has a "cain" in it on chickens, like lidocaine.
I don't think they are in pain, apart from getting their bums washed and having had the poo on it, I believe the redness and such will resolve fairly quickly.
I think the oil would help with the redness and swelling, much like desitin on a baby's bum, a barrier of sorts to allow for healing with the added benefit of being on the feathers surrounding the area so the future poo doesn't stick to it. I have also read that others have trimmed the feathers around the area, that one takes a steady hand on a wiggly chick or a helper.
