advantage kills the fleas but won't do anything between treatments if the dog is getting exposed to fleas between treatments.

doggie daycare should have a 'treat your dog for fleas or he can't come to doggy daycare' similarly to how schools have a 'treat your kids for headlice or they aren't coming to school' policy. i'd try having a word with them about this. they should inform the people who take their dogs of the need for them to treat their pets and ensure that all dogs are treated.

the best plan would be for them to treat all the dogs at the same time (e.g., the doggie daycare could offer to treat all the dogs at the same time as part of their service to ensure that the pets are flea-free).