I think that's a perfectly reasonable request. When PTOToday had the marketing agreement together with Amex to do this kind of night, I remember that a number of schools posted something about how the financial services advisor had offered to provide refreshments for the event; babysitting would probably be cheaper. You're right; you are dumping a bunch of potential customers in their laps, and the babysitting fee would most likely be an inconsequential marketing expense for them.
BUT--some may not do want to do it because of potential liability issues. You might be better off paying the babysitting fee, and asking them to donate refreshments.
When we did our event like this last year, we asked volunteers to provide the refreshments, and we paid the babysitter. But we budget for babysitting fees for our meetings and events throughout the year, so it was kind of a non-issue.
We are hosting an evening where experts explain the various plans/vehicles that parents can use to save for their elementary school childs college.
We are providing babysitting for this event to allow parent focus. Is it reasonable to ask the experts to pay for the babysitters since we are giving them aroomful of potential customers?