I've lost 8 mindees recently due to school, being old enough to go home and 1 mum gave up work but I've managed to fill spaces...PHEW!

Now I've got more part timers so it's been difficult fitting them all in like a big jigsaw piece, I've been used to full dayers ....anyway I'm seeing problems already with part timers. 1 mum has now asked if her 12m old can do an extra day from Jan 2017 820am-4.15pm. I'll have 4 EY's children but all manageable and allowed under continuity of care but I also have 3 under 8's to pick up from school at 3.15pm which will work as 1 EY child goes home at 3pm.

But my question is:

I have 1x3yr old contracted 8am-3pm on a Thursday. I set off at 3pm for school and as I have the 3 other EY (19m and 3yrs and 12m) and 3 under 8 to pick up from school (7,7 and 6) what happens if Mum is late...stuck in traffic etc. How can I pick up the 3 schoolies if I still have 4 EY?
Do I say no to the Mum of the 12m old or say I can only do it until 3pm on the off chance that the other Mum is late picking up to cover myself on having too many children? I know I definitely can't have 7 under 8yrs under no circumstances.

Nothing is ever straight forward