@Brent1 My impression is that the only reason these guests would be concerned about the radiators being cold, is if they feel the room temperature in general isn't warm enough. (24 degrees seems warm to me, but I don't know what climate these guests are used to).
You could offer to raise the thermostat for the remainder of their stay, if you're amenable to increasing your energy usage for customer-service purposes. Or I suppose you could appeal to their conscience and explain that your system is designed to make your properties more energy efficient and sustainable, to reduce the burden they place on the planet their young child will inherit. But either way, the best way to demonstrate that you're attentive to their needs would be to pay them a scheduled visit and "make sure everything is working properly."