You would be classed (well certainly here in the UK) as a vulnerable person who needs special shielding from the danger of catching COVID.  Such people here are to stay at home for 12 weeks and not to go out for anything - and preferably to get food deliveries so that they (or their partner) does not go to the supermarket.  There is no question of the SKIDs coming over in the circumstances, if they are sick. 

It doesn’t matter if she understands the concept, what matters is your DH understands it & tells her no swaps himself. I never get involved with visitation arrangements for my skids, but with this I made it very clear we weren’t having the boys because I wasn’t going to risk my child’s health, or theirs. BM could kick, scream & throw whatever hissy fit she wanted, but not a chance on gods green earth was she going to decide on the health & well-being of MY child. I told DH to keep our child safe, or I would. If you haven’t had that conversation with your DH you really need to, this is not the time to be letting incompetent exes call the shots. Your health is more important than her need for control. 

Given your fragile health and knowing that they can be contagious long before they start showing any symptoms, they shouldn't be coming over at all.

if necessary they can make up the time down the road.  When my bios were ill, I'd postpone visitation until they were over whatever they had.  Then make up the time down the road.  It's just common sense.

Chef would NEVER do this as the Girhippo was only too happy to dump off her feverish, projectile vomiting illness-laden brats.  And Chef was always pining for more time to spoil them.  Lose/lose for me.

Both of us would invariably catch what they had.  The only up side was that when the ferals were ill, it was the ONLY time they were quiet and relatively well behaved.

Again, you have a DH problem if he won't say NO to the BM.