I also think this sounds very selfish and thoughtless. I certainly don't get the impression that Misdee expects her parents to help out, but, having asked them, what she's upset about is the guilt trip they then lay on her and/or their attempts to wriggle out of a pre-agreed arrangement by asking if someone else can help etc.
Quite possibly, it sounds like they don't want to help at all, but don't know how to say "no". They then backtrack - via the end of a phone by the sounds of it, which, to my mind, is far worse than dcelining to help in the 1st place. It's that behaviour which is unfair and places extra pressure on Misdee.
Whether or not parents should help is another matter. In an ideal world, it'd be great if families were always there for each other but some people just aren't like that and have a "you made your bed (even if you didn't) you lie in it" attitude. I have been desperate for help as a single mum in the past (juggling work, illness, lack of childcare, needing to be several places at once), yet my mum would never ever offer any - even though, the maximum no. of times I would have been desperate enough to try and approach her probably amounted to around a dozen in sixteen years. In the end, I gave up telling her about my life and that of her grandchild, as the hurt I felt when she failed to sympathise or (very occasionally) help made me so upset.
But that was within an average single family situation ....... I didn't have the worry of children with SN, or the huge ongoing worry and practicalities of also looking after a seriously ill partner. I must admit, that in those circumstances, I am rather agog that it seems beyond your parents (plural what's more, so they can share the effort) to look after two of their grandchildren for what, a few hours ?
I honestly think that with my mum, she simply didn't (and doesn't) know how to deal with anything which isn't quite "right". Me being a single mum, having an abusive relationship, losing my home at one stage (by effectively being conned by my then husband) - and all the resultant effects of all of that, wasn't what my mum was used to. She doesn't do emotion, and therefore, I believe she failed to help me when she could have done (the very odd day of childcare, or collection from school) because me asking for help, was, in her eyes, symptomatic of all the trouble I'd "got myself into" in her eyes and she didn't want to be involved.
Misdee, I know your situation is different, but you still need help. I just wondered if somehow your parents, like I think my mum did/does, are actively trying to distance themselves from a situation they find frightening, or outside their experience. By not helping, they can pretend it doesn't exist. On the other hand, I suppose they could just literally be selfish and feel resentful at their retirement being "spoilt" by the demands of their family.
I don't know if that makes sense, and I apologise if I've offended you in my thoughts about your parents ...... I'm simply trying to make sense of this very hurtful attitude they appear to have. I certainly don't want to try and excuse them though ........ after all, from what you've written, it doesn't sound like you expect a lot from them, and most people would want to help out their nearest and dearest if they were as pressured as you are. It's not even as if you're asking them to help so you can go out on the town.
Practically though, like the others have said, I think for your own peace of mind you will have to try and leave them out of any help needed. A) It always seems to come with the cost of you being made to feel "guilty" and B) You can't trust them to keep their word. Accepting their "help" if it is grudging and unreliable will only create more problems in the long run.
I really feel for you - bloody parents.
Listen, I hope you don't think this is "odd" or anything but I live in North Herts and am a SAHM at the moment with an almost 3 year old. Whether or not you think I could ever be of any help, whether you'd want to trust me (obviously we can meet up, call whatever), can I just say that if there is anything I could help with, I wouldn't mind you asking (even if it's practical stuff for example, like picking things up). Can't guarantee I could always help but you'll get a straight answer. Anyway, the offer's there.