There are those parents who go right ahead and write their children's names on the response card, some doing it intentionally, believing they ought to have been invited in the first place (untrue); others truly believe the children are included. Whoever is hosting the wedding may call immediately and explain in kind terms that the children are indeed not invited. Give the benefit of the doubt, and keep the focus on who is invited, "I'm sorry if there was any confusion, but the invitation was only for you and Henry. We hope you can still attend." Stay strong but kind if they push; making an exception won't be fair to other guests who respected your wishes and found childcare. If this results in an angry, "then I'm not coming either," so be it. The breach of etiquette is theirs, not yours.