Doesn't matter if it's a llama. The answer is the same regardless of the parent who asks.
"Hi can you collect X along with yours today, and I'll be over later to collect them?"
"Sorry, no, that doesn't suit me today." (If you don't want to just leave it at that - although that is perfectly sufficient - you could add something like:
We have an appointment to get to after school.
We already have plans that I can't change.
I have something else I need to get done this afternoon.
You don't need to say exactly what those plans are - just that it doesn't suit you for your own reasons.
You could offer an arranged afternoon another day, IF that suited you. But don't feel you need to.
Just don't always be available whenever someone else is in need - you have things to do sometimes, places to go, appointments to keep, bookings you have made for fun stuff, need to go to the shops for something that DC needs, plans to meet with relatives or friends after school.....all sorts. So you aren't available at the drop of a hat for others who can't organize their lives and have childcare arrangements in place.
Assuming that you are either working PT or SAHM to be able to be at the school gate daily, you have made sacrifices to have that flexibility, I assume, and you don't need YOUR sacrifices to become beneficial to others when they won't either make similar sacrifices themselves or pay for proper childcare arrangements.