@jamandmarmalade I loved your post! Take compliments where you find them
Thank you for the "honourable mention" as it were
@SummerBluez op gets a LOT Of help and doesn't seem to really appreciate it or even consider that even if her parents are saying they don't want her to reduce how much they do with it affecting their health it would be the kind thing to do I think
I'm probably closer in age to the grandparents in this scenario at 49, I absolutely CANNOT do what I could when I was a nanny/childminder/dd was little. Admittedly I'm disabled but I talk to friends and family with these sort of commitments and they often daren't say to the parents "it's a bit much" - sometimes as they worry that there will be more practical or financial pressure on the parents, sometimes because they fear the parents or at least one of them will take offence and use the children as pawns - which I've seen happen too sadly. I'm afraid I can well imagine the op falls towards the latter category.
I've seen the heartbreak of "well if you don't want to be a PROPER granny that's fine we won't see you at all then!"
My generation mostly I feel and certainly my mothers were expected to just get on with it! And if they had dc then they were the ones had to consider childcare - either paid or work part time/sah but then the other parent had to be earning enough to facilitate that.
I say certainly my mothers as I feel hers was really the first generation where grandmothers were staying in work in their 50's and 60's and beyond - mostly out of necessity.
My mums mum didn't retire until 65 by which point half the grandkids were past needing childcare and I was about to leave school!
My dads mum LOVED her job and didn't retire until her 70's by which point almost all her grandkids were teens and older
They simply weren't available to do anything but occasional/emergency childcare
In addition my dad was army so we weren't living near family or even close friends. There wasn't any choice except paid for and mum worked or she stayed at home. As it was the way my parents mostly worked it is mum did jobs that were eve/nights like bar work, late shift shelf stacking, office cleaning etc so that dad watched us of an eve (he had fairly much "office hours" except if he was deployed or on a duty) if he got deployed mum would quit for that period...but in the confident knowledge that when he got back she would soon get another job, this was late 70's early 80's when uk was doing ok economically in that respect. Or she took temp eve work. Some places if there was someone mum knew well they'd figure out between them a shared arrangement and each woman would work part time and they'd work their hours around each other and reciprocate on childcare.
When I split from ex I was stuck! He was also army so no family or friends nearby (we'd only been there about 4 months!) we were in a quarter so that meant I lost my home too, I was very naive in many ways back then being a Sahm with no savings etc it was a steep learning curve I'll say! And that was with one child!
I'd been childminding but knew the marriage was failing so gradually reduced my numbers and then closed the business as I didn't know where I'd end up and if my new home would be ok for childminding. Had to claim benefits initially, got a ha house (luckily quite a nice one in a nice area) with help from (Tory!) Mp as council were being arses and saying they'd no obligation to house dd and I as "no connection to the area" but I couldn't afford to move to where my family were I couldn't even afford to visit them!
Luckily found a decent nursery for dd, got a full time job and booked her in at the nursery and then with the nursery hours I used to have to leave work on the dot every night (that went down well!) and belt it to the bus (ex had grabbed the car in a "midnight raid") soon as I got off the bus, belt it along to the nursery to get her in time. I got very fit! 
I was knackered most of the time it was very much a grind cracking on with it all. But dd was my responsibility and ex was fucking useless he was too busy pretending he was a born again teenager going out clubbing and to gigs and concerts at all hours and barely making it into work most days, hungover/drunk all weekend etc
Just as dd was starting school I went back to uni, the aim being to develop a career that would work around her, unfortunately that idea was kiboshed when I ended up hit in the car by some numpty on their phone! Hence disabled now.
I hope op has contingency plans for her and the 4 dc in the event not only of a split but if oh becomes incapacitated or dies or if she does...you never know.
I didn't expect to be a disabled single parent at the age of 35!
She is VERY lucky to have all the help from her parents but they are able to do less as they get older and health starts failing.