Hypothetical situation at the moment but may arise in the future due to job changes.
Would you feel happy to leave a sensible 9 year old DC for up to 1.5 hours after school on one day a week? She would also be walking or cycling home by herself/with friends (though the journey home alone is by itself not an issue as she would be doing this on other days as well).
Potentially her 11 year old brother would also be there (and could meet her en-route home if needed).
One parent works 15 minutes walk away (so no delays in getting home and close at hand). DC knows various neighbours she could go to in case of real emergencies.
Feel like 9 is just the "wrong" age - a couple of years younger and we wouldn't consider it, and a couple of years older and we wouldn't worry at all!