@Hannabanana2021 ohh firstly I'm so sorry about the loss of your babies 💐 I know those surgeries of old and if it's what I had to go through, it's total hell on earth.
As PP notify the nursery and ensure the message is passed back to mum in a formal way. If she kicks of you can say we did xyz.
Covid is such a hot topic because everyone has their own risk assessment on it and I feel for you because in part your hands are tied so you have to say "well we don't agree with this but it's our only option" and you may have to bring him back and risk him getting covid (btw I'm very risk averse re covid re having it myself so you have my total sympathies that this isn't what you would want to do) there's a different risk level to going to tescos for weekly shop to knowingly entering in a house with positive covid people in it.
Your DSS no wonder he's nervous with the antics of mum and SS being involved. SS don't just arrive for no reason and nursery's don't log issues for no reason either.
I know you said you could contact her but I think for your MH I wouldn't. I get the pull 100% but I wouldn't.
Do you have any external help you trust that could help get you some rest ? That DSS would feel comfortable with or you could get DSS comfortable with. Would he feel more comfortable as a group with all the kids going somewhere (keep them in a little tribe unit ? When granny/whoever looks after them all)
Courts I think courts is only way forward I'm sure that a judge wouldn't look well on this behaviour... get everything outlined out how to handle holidays, festive seasons, normal contact and illness.