I still think there's some confusion here. Most people are just going to mail in their rsvp. You only called because of the issue with the kids. When they mail in their rsvp they won't know anything about this. How many people are going to come to the wedding prepared to pay for their dinner. None I would guess. When they eat, dance, give an expensive present, etc is the mother of the bride going to stand up and say... excuse me, you all need to pay for your dinners now. I don't really think so.
I can understand if they had written on the invitation, if you choose to join us for dinner it will be at your expense. Then people could make their own decision.
I think you are getting misinformed. I'd call the bride or check with some other folks who are going to this wedding. I really think you are mistaken here because there is no way it could be handled after people eat. They'll be standing at the hall collecting 150 people's $70. saying to the server presenting the bill, "Wait we didn't get the money from 15 people yet?" Or, sorry, we're short $300., some folks didn't bring their credit cards? No that couldn't happen, really.