She's an old friend of my mum's (mum now deceased). She has been in and out of the picture for decades, even though we lived overseas and so didn't see her for years at a time; she kept in regular touch with my mum and always remained a 'current' friend rather than a past one, so to speak. She lives in the same city as me and my DH/DC, and she is a kind person who frequently offers to babysit, come round with presents, sends little nonsense messages on Facebook, that sort of thing. She has one DD of her own (who is very nice and no-nonsense), who has now gone off to work in another city.
The problem is, none of us have ever really liked this family friend. My mother used to say 'Oh God, it's her - I'M NOT HERE' and run away if the phone rang, so me and my sister had to make awkward conversation with her. My mum found her overbearing and intrusive and really wanted the relationship to just die, but was a bit passive and 'what can I do?' about it all and so it's just kept going for decades. I had to invite her and her DD to my wedding because DM insisted I couldn't not; DSis had to invite them to her wedding because DF insisted she couldn't not (DM had died by then). She means well, but none of us actually like her at all.
So my issue (which I'm aware I'm very lucky to have): she keeps on at me about wanting to meet up, and has always been desperate to babysit my DC (now aged 7 and 8, both boys). I did take her up on this a few times in the past (say maybe 5 in total over 8 years?) and the occasions varied from her taking them out for the day to her sitting for them when they were in bed. I preferred the latter as frankly I get the impression she doesn't like them very much 
she used to be a primary school teacher and her own DD has always been good as gold, while my two are not so much (we're working on it, believe me). She has always just sounded so cross teacher around them, even when I'm there, that it got to the point where I didn't really want to send them off alone with her.
The very last time she looked after them was the final straw for me. She begged to do it (please note I only actually instigated these occasions on 4/8 times; the other times she's insisted pretty much) and it ended up being a massive deal. On the face of it she was only sitting for them on Sat evening while we went out, but: she needed dinner so arrived early and ate with us (we felt obliged to cook), we had an obligatory hour's chat before going out, we had an obligatory hour's chat after coming back, she couldn't possibly drive home at that hour so stayed the night, she stayed for breakfast and hung around hopefully until half 11 before realising that was it and then headed home. I don't want to do all that again 
Basically I have tried to go low contact and be bright, breezy, unavailable since then (last summer). I haven't taken her up on the many babysitting offers, I've been polite yet unavailable,. I just want her to take the hint 
She's now messaged this morning asking if she's done anything to upset me. I can't work out a way of responding that will leave us at low contact but not cause massive offence please help!