I tend not to be as busy during school holidays now, but certainly have in the past, and regularly had that many children, and covering a wide age-range.
I think it is absolutely essential to have some kind of daily/weekly planning in place -nothing heavy-duty, just deciding exactly what messy play activity you would do each day for instance.
Anything you are going to do needs to be sorted out before any children arrive for the day, as you will not have time to do anything once children start arriving.
I always ensure that older children have free access to games, craft activities, construction sets etc. that need minimal supervision in case you get tied up with nappy changing/distressed toddlers etc.
I always tried to have at least one day out to a farm/safari park or similar each week, and definitely went for at least one walk a day, usually to the local park or woods. We liked having lots of picnics, even if it was in the back garden.
It is not always possible to suit every activity to every single child you have at the same time, so I made sure that if we did something more geared up to the younger children in the morning, then I would organise something more suited to the older children in the afternoon.
I also got my lunch ready before starting work for the day, and had juice in the fridge as I never had time for a coffee!
During the summer holidays, I finished every single day with a headache and totally exhausted, but know that I had done a good job and that the children had all gone home happy.