TopsyandTim - There are any number of reasons why your relative's employer has put the children into a breakfast club, but as you say it was just for one day, which suggests it was a trial - either they wanted to give this a go, or the children wanted it - and it may not have worked out. BUT, it is their decision to make, and your relative will obviously reach a point in her employment with this family when she is not needed because the children are growing and may start to stay for after school clubs or go early for other clubs, sports training etc.
It may well be that your relative's employers are re-thinking on the grounds of her chronic illness, for instance, what does this mean for them as employers, might they be worried that at some point in the future she might become worse and blame it on them? I'm not saying that this is a thought or judging it, but if my cleaner told me that she had just developed a second chronic illness I would definitely be considering whether anything in her 'job description' might aggravate her condition and I wouldn't want to be in that grey area two years down the line of a dispute about whether her bending to get in a low corner had provoked some inflammatory reaction, and that I might be responsible for it. So, sad as it may seem, your relative's chronic condition might well be something that her employers are weighing up and you do say that she gave them her all during the pandemic - if she is letting them know that, and then talking in the next sentence about her chronic condition, that in itself may be the trigger for her employers re-thinking their childcare arrangements. On the other hand, if it was just the one morning, then it's just a toe in the water. Maybe your relative should look for another job anyway, and this time flag her two chronic conditions and get everything out into the open from the start, i.e., what she is capable of doing, because it sounds as though she's allowed the boundaries to slip during the pandemic.