Maybe if you'd waited to have kids till the "40 plus mumsnet age" rather than shagging a squaddie when you were very young, you would have had a more solid financial foundation for your family and could have been a little less brittle and black and white when it came to sick days?
I am resorting to being offensive to you, OP because you're being rude to others.
"Sick" in quotes means you don't think other people's children are really sick enough to stay off school. So what? Doesn't hurt you if some people take gentler care of themselves than you felt able to take of yourself. Except it does hurt you, doesn't it? And lashing out at us all is your way of processing something. Sounds like you have had a hard life, and maybe others now make different choices. If you accept it is ok they stay home, you also accept your own life wasn't fair. You would need to accept that if you and your kids felt a bit unwell, you had to suffer. You didn't have inner permission to be soft with yourself or your children. You didn't have outer resources. That is a sad thing. But it's better to sit with it than create a story about others. Your superego is trying to protect you from feeling those feelings, by keeping rigid and asserting everyone else is deficient.
"Lucky" means you believe you're not lucky to be healthy, you believe you have either better genes or should be praised for your work ethic. Again, here you are trying to process the fact that your grit, and determination to get to work, something you've always prided yourself on and expected to receive approval from...maybe was only just luck all along?? Maybe you didn't even feel as ill as others on those days when you pushed on and they stayed home... but you did feel ill, didn't you, sometimes, and it was still unfair you had to go. If you admit you were lucky, you're letting go of your real pain when you were ill, and the part of your personality that prides itself on pushing on.
Also, you want to be angry with others, because you've always got yourself through the hard times by thinking "well at least I've got my grit". It would be a shock to realise that maybe those sickly people have courage too. Maybe (whisper it) some of them have more than you. Or more than a soldier in combat, even? Who can imagine? Who can compare?
After writing all this I genuinely wish you well OP and I wish you softness, for you and your DC and DGC.