Ell your dh and mil that they are being incredibly selfish and can't love your dc at all as much as they profess to if they are willing to expose them to mil's bugs. Just because they've had the bugs before, there's no guarantee that their antibodies will protect against whatever mil has. After all it was serious enough to put her in hospital but it didn't put you guys in hospital so it might have changed into something a bit different. Might not - but not worth risking your dc for!
And mil upset - well tough, she is poorly and she doesn't get to chose to inflict it on your dc. She can speak to the dc on the phone for a minute it two and catch up. She doesn't need to have them there, bored, at her house for hours in order to catch up!
In this instance I'd still stop even the dc thatwants to go with your dh from going, because any sensible parent wouldn't want to risk their child being ill, especially if they have already been ill recently and thus could equally well be at risk of picking stuff up more easily than usual.
And if either mil or your dh complained, I'd just point out that if they loved their children/grandchildren they wouldn't be exposing them to the risk of being very ill again and that to demand they go is selfish and shows they don't love the dgc. Or indeed that mil can't love dh that much as she is happy to risk him being ill too, she could speak to him on the phone if she genuinely wanted to catch up without putting anyone at risk.
And if dh kept on I'd just start every reply with 'oh don't be so ridiculous. Of course we can't go to your mum's and risk being I'll again etc etc'.
This is one of the few times that you have a cast iron reason not to visit your mil (assuming she doesn't have stomach bugs very often) - nobody normal and sane thinks it is a good idea to go see people with stomach bugs just because they want you to go - particularly as you don't need to take supplies or do anything to help them. If your mil rants to her friends they are not going to agree with her this time that you should have gone whereas usually they will probably agree with her.
This is one of the few times that you really really shouldn't go - and if you do give in and go, then you are handing them a great big victory in the 'every Sunday to be spent at mil' war even though you may not realise it.
Take a deep breath, say that whilst you can't stop dh from going, due to mil's illness it is not reasonable to go to see her and thus neither you nor the kids will be going. If he complains, just say that if he loved you, he wouldn't even ask you let alone expect you all to go. And then repeat, calmly, every time it is mentioned.
Of course it's not going to be easy but you know that already so you can steel yourself against it. If dh gets angry or sulky etc then call him on it. Tell him not to be ridiculous, to grow up and think like a responsible parent not a sulky teenager, to stop punishing you by sulking - show him that you know what he is doing.
If you want a 'carrot' to dangle then say that you will all go to visit when mil is properly better. But not if you are forced to go now.
(apologies if I've cross posted or thread has moved on lots since I started typing this a couple of hours ago and ws then interrupted. But doing it on my phone and didn't want to lose all my typing!).