I once agreed to do this for another mum at my children's school, three days a week. They lived near us and the mum, who has MS, would only have a thee minute walk to collect her child from me rather than 20 minutes to the school.
Except after committing to it, I realised how rarely we went straight home from school. We often went straight to GPs from school, or medical appointments, dentist, the park, after-school activities, or just to Tesco for some shopping. That's when I discovered what a nightmare the child was out in public; who'd happily roll about in the aisles like a tantruming toddler, pull stuff off the shelves, demanding to use the baby swings even though he was 8 years old, etc.
When we did go straight home his mum took longer and longer to collect him, and sometimes I'd end up feeding him as he hadn't been picked up by the we were having our meal. The lack of table manners was the last straw, as my DCs were at first astonished then started copying. I lasted four weeks before I had to say I couldn't manage it any more. I copped out a bit, saying I had too many commitments on those days rather than how awful her son's bahviour was.
Don't do it, OP, as in my experience you never know exactly what you're letting yourself in for.