We thought we might have to do this a few weeks ago. DH has 'underlying health issues' and I was recovering from an operation.
DiL became ill with obvious C19 and son felt 'under the weather'. Then toddler was 'not herself'. Son (working from home) told work he was taking time off sick, and went to sleep in toddler's room. They muddled through, as toddler was quiet and sleepy & son only mildly unwell. He was able to monitor DiL who felt dreadful but not breathless.
We monitored them by phone twice a day. A friend of DiL's who lives alone, heard about this and said that as she was young and fit, she would go to care for them if need be. We were so grateful,but it wasn't necessary.
I suspect the 'back-up plan' is that someone breaks the rules. I imagine that local authorities have some provision for taking a child from infected parents into care, but most of us would break the rules.
I should add that of course, none of them was tested! Also that son & toddler took about a week to recover, and DiL 2 weeks.
