It sounds to me as if it’s not really this event which is the problem. For what it’s worth, I wouldn’t have wanted potentially infected with a vomiting virus children visiting my house if I had children there, and especially not if I was about to go into hospital. The possibility of spreading norovirus into a hospital would be something I’d consider seriously. I am not of the same opinion as you regarding vomiting viruses. I feel like colds are completely unavoidable as they are so common and so easily spread. Vomiting viruses are marginally less easily spread, rather less common and way more revolting, so more precautions makes more sense in my head.
That said, if you were in an equal child-sharing situation with your ex, the situation would be completely different. If you had them 50:50 you could agree to rules round sickness that would work both ways. You might agree that in the event of certain sicknesses, the children would remain where they were until it was past and it would even out over the years as it would happen both ways.
It also sounds like your ex and you share the same attitude to sickness, but his new partner either doesn’t, or doesn’t want your children there and is using any excuse. If it’s the former, it doesn’t seem unreasonable, if it’s the latter, it obviously is. It’s very difficult to tell what’s in someone else’s head and unfortunately, you’re in a situation where there’s a lot of ill-feeling, so it’s even harder to judge.
What I would say is that you are understandably protective of your children and unhappy that they are being let down, and from that point of view, I think perhaps you need to protect their feelings by telling your partner things well in advance. Maybe I have misunderstood, but the impression I have is that you hadn’t told your ex about the vomiting bug until he arrived to take the remaining children. If you did and he still didn’t talk to his partner until he was on the way home, then the change of plan is entirely on him. But if he didn’t know you had sickness in the house before he arrived, then he didn’t have the chance to let you know that would change his plans for them. If he had known, and had discussed it with his partner in advance, then you could have managed their expectations and perhaps he could have bought a picnic before coming, so he could have taken them to the park and fed them there. I wouldn’t take any potentially infected children out to a fast food place either as unfortunately vomiting viruses can come on suddenly and there are few things more grim than being out for a meal when someone else’s child vomits. I understand it happens and can’t always be predicted, but if you know there’s a high risk, it’s fairer to everyone else to minimize it. I know they go to school, but missing out on burgers is not the same as missing a couple of days of school.
So I clicked on YABU, but I think it only applies because it’s a vomiting virus and we obviously have different opinions on how those should be handled. But in a wider sense, you absolutely are not being unreasonable about how your ex is treating his children. He’s an asshole of the highest order and should be treating his own children much better.