Interesting how, upon being challenged for exploiting your staff, you have changed your mind and realised that, actually, you give her two days off a week, not one! And, actually, you 'may' go out on Saturdays, not, 'whenever you like', due to having childcare whenever you want! Funny, that.
You say that £15.50 a day is all disposable income (as if you think that is a great deal), but that rather ignores the fact that she has no home, and, presumably, almost no possessions as she exists in a small bedroom in your flat. She has no home, nowhere that is homely, no-where to entertain or meet friends. You refer to the 'helpers' meeting in the park. Don't you realise that that is because they are paid so poorly, they don't have homes to invite friends around to? (whilst their employers pat themselves on the back for providing free accommodation and use it to justify poverty wages) Presumably most of these helpers are meeting whilst they are working too, so not even meeting in their own down time (not that they would have a lot of money to socialise with....).
Buying little gifts, or any discretionary act that you consider kindness, but are not obliged to do, is no substitute for decent pay and terms and conditions enforced by a contract and hence not discretionary.
A contract makes give both parties equality, as both have rights and responsibilities, It's deeply patronising to leave someone in the powerless position of you 'choosing' to be 'nice' . it is delusional to think this makes you a good employer, whilst her basic contracted conditions are so poor.
You have referred repeatedly to have 'fond' you are of your help: its almost as if you regard your fondness of her as a perk of her job. People use the word 'fond' when they are referring to pets, small children and chocolate. It is really telling that you use it to describe her. You're attitude reeks of patronisation. You have clearly 'othered' her and don't regard her as an human being of equal worth and dignity.
She's a middle aged women (if she has worked in this job for 30 years as you say) who lives in a tiny bedroom, has to socialise in a park, works 6 days a week (or 5 if you want people to stop thinking you are exploitative!) for £15.50 a day max. There is no way you can reframe this to be anything other than exploitative. She has to put up with this suck, and being a tool for you believing you are nice due to your discretionary acts of charity, due to her poverty.
Really, your attitude sounds like something from a couple of centuries ago.