"I could, but I don't want to".
Channel your inner Phoebe Buffay.
I can't sorry. I'm swamped.
Sorry, I haven't got the car that day.
Sorry, I've got other comitments.
Sorry, I just haven't got the time.
Sorry, I've got a hundred errands to run and I won't be home.
Sorry, no, I've got family stuff on. (could be baking with DC, taking elderly sick person to GP appointment, xmas film and hot chocolate in your onesie - doesn't matter!).
Sorry, I can't have X child over for the afternoon, DC has got very obsessive over their toys/is on a screen ban/has extra maths work.
If they push, take out the 'sorry' part of every answer. If they persist, say 'I've said no. I really have to go now it's dinner time/bath time/gotta hoover up dog vom...' etc.
Any insistence over whatsapp is easy to ignore.