In legal terms, nothing much - unless their contracts of employment support their view, or they are in some way different from the rest of your employees, in which case they might be able to claim they were being discriminated against.
In more general terms it does not seem like an excessive demand, but if you feel strongly about it, how about aiming for a compromise at time and a half? Or asking for volunteers instead? Or putting it to all your employees, and asking what they think would be fair? This is the kind of minor grievance that can blow up out of all proportion: since you have been given early warning, take it.
