There's nothing wrong in asking a question related to a child's health- we're nurses, and (most) parents understand that we just care about kids and want to help. Unless I was VERY sure, though, that a kid had bed bug bites, I'd start the conversation with something general, like "I was asked to check out your child's right arm and noticed some red bumps on her arm", and let the parent take it from there.
As far as neglect- bugs are hell. Lice, scabies, bed bugs...whatever they might be, they creep, they crawl, and they are pain in the patooty to get rid of. As long as I know that a parent is trying to solve the problem, I wouldn't worry. If the parent wasn't doing anything about it, and the kid was getting open skin wounds, couldn't focus due to the itching, etc., well that's not ok. I doubt a parent would like to feel that way, so why would their kid? At that point, some close follow-up, parent conversations, and discussion with the pedi would all happen first, and then maybe I would think about a CPS call if all else failed. But that should be a far-down-the-line move to make.