I used to do the school run a lot for DHs kids, DSD and DSS. They are here the majority of time, just over 50% at 4 nights per week and with their Mum for 3.
I used to do the school run on both DHs days and their Mum's days regularly because my work is down the road from the school and it was easiest out of all of us for me to get to work on time when dropping off and quickest for me to get back to them at after school club after work.
I'd say (outside of Covid) this was going on for about a year and a half. It started as me just helping out a few times and gradually just became the norm.
I recently had a baby, she is 5 months old. I haven't done the morning school run since. She is a bad sleeper and I'm knackered basically, mornings is actually when she sleeps the longest chunk so I am not getting up and dragging us both on the school run. DH and ex seemed understanding at first but I can sense this is waning now and I keep being asked with by both, occasionally with a slight tone of annoyance and sort of "why can't you". It's been stressful between them trying to sort it themselves for the last 5 months I think but quite honestly I don't care.
AIBU to say to both that they need to assume from now on they will be doing this themselves and I won't be available, IF I feel I can help every now and then I will but they should assume it'll be them from now on.
I think my husband doesn't mind as much as he starts work a little later but I know it's a struggle for his ex on her days and she was quite relieved when I started doing this (and grateful at the time).