I’m a professional cat sitter. Cats and owners vary wildly. Some are high maintenance, requiring multiple litter trays cleaned, a variety of food presented at each mealtime, complicated water fountains to be cleaned and refilled, medication to administer, cuddles or play depending on mood. There may be curtains to open/close, plants to water, parcels to bring in, and other assorted house tasks. An owner that wants full and detailed reports and photos.
Others are very low maintenance; just dry food to be topped up so no messy bowls to be washed up or uneaten food to be disposed off, no litter tray, and a cat that isn’t interested in any interaction, and an owner that doesn’t want welfare reports.
Your situation is clearly closer to the latter.
I charge the same regardless, just to keep it simple. £12 for two visits a day and £9 for single visits. But as your situation is a quick easy one, I think £20 for a 14yo who’s happy to do it is absolutely fine. They’re not a professional, they won’t have insurance and a parent is the one with the responsibility for dealing with any crisis.
The only thing I would say, is please make sure he can wash up the bowls as you’re feeding wet food. It’s quite important. And water should be fresh - when you say ‘topped up from a bottle’ do you mean you fill the bottle before you go and that’s what’s used all week? Please ask the kid to empty the bowl and refill from the tap every day. If he’s only across the driveway he can do this from his own kitchen if necessary. Fresh water is a basic essential (I know they drink from dirty puddles half the time).