Sorry your DC is ill.
Regarding WFH, I don't think you're being reasonable.
WFH is a relatively new concept for some of us who raised children and worked full-time. I wouldn't have been allowed the option. I'd have had to take dependent leave in order to be able to stay home and monitor illness. Otherwise, my child would have been ill with no supervision.
But, unless your child is constantly ill or incapacitated in some way (which I would hope was not the case with chickenpox or you'd go to the hospital), WFH shouldn't be that much of a burden, surely?
Don't you just check-in, or leave them in the same room with you so you can be there if needed?
Not sure what your job is, but I can't see that your DC would need you hands-on 100% of the time with 100% focus at 5 years of age?
When I had chickenpox at that age, I was on the couch, watching cartoons with chicken soup and taking fever meds as/when needed.