This sounds like a lot especially with your husbands illness. No wonder you're feeling overwhelmed. Ive got completely burnt out a couple of summers so I can see where you are coming from. Here is my survival advice. Please also know that you are worth looking after too, infact its essential that you spend a bit of time on you. So however that can take shape, it needs to happen.
1)Paper plates. By a big pack.
- I find a big wall chart helpful. My children otherwise ask constantly what we are doing and I rhink argh I dont know!!. Usually im saying Im at work and your here /there but it helps them to see the nice thjngs coming up and means I feel more settled that we have a plan. It feels like it lightens the mental load a bit for me doing it in advance.
-1 lots of children will be using nursery less over summer is there definitely no way the nursery can have your dd back even for a few days over the summer? If you explain situation with DH I would imagine they would be understanding.
-2 who are your DSs friends? Firstly is anyone he really loves going to a holiday club? If he goes with a best friend he might not hate it so much. Secondly plan in some play dates.
-3 Who are your friends? Who can help out/be company? Even the shit overwhelming days are lighter with another person to eyeroll with. Do you have anyone near by?
-4 I used to hate this when people said it to me but it is true you can adjust the kids expectations of you. Morning I havent cracked but when I became a single mum my kids learnt not to be so demanding because I physically couldn't jump up and down 1000 times a day anymore with no back up/help. Im able to take some quiet time now to read, yes it means im slower on a chore or two and yes sometimes they are sat with the TV for that time (although often they will make their own entertainment which again is a new skill because ive steeped back a little). I a thousand times more chilled, happier, mum because I make a little space for me. I spent years feeling too guilty to.
5- Routine. Kids love it and its reassuring to us too. Even just every Thursday we go to a park..little buts of pattern give shape to the week.
6- If they are crabby add water.
7- is there options for your husband when you need to spend time at home? Noise cancelling headphones or is there family near by that he could have a restful day at occasionally?
8- What activities with the kids do you like best/ what places make you happiest? Go to them.
This is a really tough situation to be in. But I think you can still build a holiday you'll mostly love. Try to let go of the childcare guilt. I think a day a week/ fortnight to yourself would do you the absolute world of good and be a worthwhile investment if your son can go to a club with a friend and your daughter can be at nursery.