I think, if you have had a child with someone, your husband isn't going to be shocked by the presence of an engagement ring, whether you mentioned it or not.
You kept the ring (some would say you should have given it back) so it's yours. You don't wear it, so sell it. Might actually HELP if your husband sees that you're getting money for it. I agree he will see it means nothing.
You might consider downplaying the fact that the ring was from a failed relationship. Sometimes buyers don't want to "jinx" a new relationship.
You could also consider selling it to a reputable jeweler. No security risk, and you don't need your husband there when the buyer comes around. Your husband wouldn't even have to know about this transaction either.
If you have a gut feeling though, you shouldn't completely ignore it. There may be more here than you've shared.