I'm not in quite the same situation I only have 2 DC, one at Kindergarten and one at school (we live in Germany) Kindergarten always have something going on, DD needs to take this, or bring home that, appropriate clothes, waterproofs, snowsuit, winter boots, wellies, all have to be taken on the day brought home and sorted. DS has early days and 'late' days either 07:40 or 08:30, has to carry literally everything around with him, sports kit that has to go in on certain days, activities, a never ending list of things after school. My problem is that all the information that tells me this, is all in German - and my German is still really poor, so absolutely everything has to be typed out into a translator.
For the regular things that must be ready to go on a certain day, I track back to when it has to be washed and goes in a day specific washing basket. So if I need it ready for Monday, it goes in the Friday washing, you get in to a habit with it and it becomes easier. Letters go in a red 'in tray' until they are translated, then go in a folder, school, kindergarten, activities etc etc, everything goes on the calendar. I have a organised in date order memo board for irregular things which is linked to the calendar on my phone which I can set reminders for.
I'm an organised person anyway, but it helps me.