What should I do?

So me and my partner have 7 children between us my 2 from a previous relationship, his 4 from a previous and our 1. We have his 4 every weekend and half of all school holidays. She gets 30% of his wages in child maintenance, we litterally became homeless last year because of it. 6 months ago she’s got herself a boyfriend and moved him in with her. Since then she’s changing things all the time. She’s making a huge scene about how she shouldn’t have to pick them up from school on a Friday as its his night (he works till 5 and can’t possibly do it and I have my own children to pick up at the same time 25miles away, in the court order it’s down that she does school runs because of his work schedule - she doesn’t work), then she started asking him to take them home 3-4 hours later on aSunday because she has plans. Well recently her boyfriends started texting us at least once a fortnight on a Sunday morning saying she’s too ill to have them back can we have them and extra night. I’ve bit my tongue and ignored it, its been 5 times now, some over Christmas holidays so my kids were home anyway and it made no difference, but now she’s starting to do it on school nights. This evening her partner texted saying she’s been super ill since about 30minutes after he picked then up yesterday, has a cough, had an asthma attack and she needs to be tested to covid so we might have to have them an extra few nights. I’m fuming, I mean surely she would have had symptoms beforehand? She should never have sent them at all if anyone in the house is ill especially with covid about (she ruined our Christmas sending them throwing up, it spread throughout our house for weeks). If she has covid and had symptoms from right after we got the kids chances are the got it to and are spreading it to me and my kids.

Anyway me and my partner got into an argument, I said I want him to take his children back tomorrow as it says on the court order, a day we are paying her maintenance for! He said he has to wait and see what they say. What do I do, I’m so annoyed, I don’t mind having his children I don’t but they shouldn’t be here spreading diseases, he has work 6am Monday so can’t watch them, and I have to take my 3 to school a 10minute drive away in a 5 seater car so I can’t have his 4 too! I didn’t mind so much when it was holidays but now she’s messing with either his work hours losing us money, potentially getting him fired for skipping days, or my children’s education. How do I put my foot down and make him take them back without being the bad person? (BTW I test myself and my children 3 times a week with lateral flow tests so know we are atm safe from covid)