LW, tell your kids. You don't have to tell them about the affair or that Marigold in particular might have a different dad. But tell them that neither you or your husband would be surprised if one of them had a different father but never knew for sure, that this was something private to your marriage that you were both okay with, that your husband was okay with it and loved them all, that you never told them because you didn't want any of them to feel like they didn't belong in the family, and if one of them does get a surprise in the kit, to come talk to you first.

If it turns out they all have the same dad, tell them they can come back and nag you about family secrets once they're all secure in knowing who they are (or go "eww parent sex I don't want to know" and never ask again.) If Marigold does have a different biodad, this gives her the chance to work through some stuff and decide what she does about it before either of you share with anyone else.