I would use the late arrivals as part of my excuse saying it has become too disruptive to our day and considering other children arriving during the same time frame come having eaten from now on the first food served will be a small snack at whatever time and you expect all children to have a full breakfast before coming to daycare. Then if you want to add that any child arriving prior to 7:30 that brings a parent supplied breakfast will be allowed to sit at the table to eat it while the other children start their playtime - make it sound like eater is missing out.

Since you don't charge any extra for the service there is nothing they can come back and say they expect a reduction in fees because it was a free service they were getting but one that has now gotten out of control based on varied arrival times.