I have a friend who sounds very like the ex-wife in terms of insisting that her children are constantly ill. Her ex-husband, who I'm also friends with, divorced her because of it and she constantly moans that he thinks she's a fantasist. But so far, in the last decade, she's claimed to have cancer (two different types which suddenly, miraculously went into remission), ADHD, Aspergers, bipolar type 1, gangrene... and every single virus going around. Her children (one with ex-, two with her new partner) have variously been "diagnosed" with asthma, autism, deafness, various life-threatening food intolerances/allergies, ADHD, that one where they don't listen to being told what to do and when to do it... oppositional something disorder, selective mutism... and every virus going around. Her latest thing is that her middle child has Tourettes Syndrome - which he was referred for, saw a consultant, and was diagnosed within days! She also boasts about buying dangerous medications from America, when her GP won't prescribe them to her willy-nilly. Her partner is seemingly oblivious, rarely at home, and is a lot younger than she is. She genuinely loves her children, or seems to at least, and she really does have some good qualities. It's just that those of us who have known her since we were all kids, really don't believe, or trust a word that comes out of her mouth. At all.
I've known this friend for years, ever since we were at primary school together. She has always been an attention seeker, and I think she realised that if she claimed to be desperately ill... she'd get the attention that she wanted. Most of us gritted our teeth and put up with it, until she started to involve her children (actually, two of whom are miracle babies, given that if she'd the types of cancer she claimed to have, and been receiving the treatment she was claiming to, they'd have been born hideously deformed in various ways - they're not!). A few of us became concerned enough to ask her mother, a few years ago, how we could support her through her cancer disgnosis/treatment, only for her mother to tell us that there was nothing wrong with her, she's a pathological liar, and they're more than aware of her habits.
Knowing how long it takes for a child to receive any sort of diagnosis (my son genuinely has Aspergers, and, incidentally, also suffers from migraines following a head injury a few years ago, which yes: can lead to hospitalisation... our longest stay was 4 days with one of his, and he's missed a fair chunk of school - but he has a clinical diagnosis, and is actually undergoing investigation into how to stop him from getting them. If your DSS is having them frequently, and is being constantly hospitalised with them, then he ought to have been referred on for not only a diagnosis, but also investigation into them), the fact that my friend was able to get them within a matter of weeks, if not days, was our first clue that she was lying - or, as her ex-husband puts it, "fantasising". We're all desperately worried about her, and - perhaps more importantly - the effect she's having upon her children, but have no clue what to do about it. Other than refer her to SS, which we're somewhat loathe to do (her children are all loved, and otherwise well cared for... it's just that she claims illness, after disease, after diagnosis for them!) And yet, in hindsight, she was always the one in our group who had weird and wonderful things happening to her outside of our sphere - gorgeous, loaded boyfriends when she was 13, for example, who'd whisk her off on dates... when she was actually sat in the church choir with at least one of us in the congregation and aware she was there, or the time when she was asked to work as a manager in her work experience company, because only she was smart enough to run the place... at 15. Her ex-husband, incidentally, has primary custody of their DC because of her issues with the truth.
So, OP, I get where you're coming from. Perhaps have your DH talk to his children's school? Because if their DM does have an issue, or a problem, it really does need to be dealt with for all of their sakes. If your DH's ex-wife is seeking attention, then perhaps the underlying cause needs to be identified, particularly if the children are genuinely missing so much school (my friend's children don't miss school, even with all their diagnosis' and illnesses!).