That isolation will mean my 3 year old is never in
If you, and only you, abide by it, yes. If most people abided by it, no. What we have at the moment is a little like the Tragedy of the Commons. (For people who have had a life before this thread
: www.britannica.com/science/tragedy-of-the-commons)
In the present climate, if you kept your child off, they'd just be deprived of their pre-school experience 5 days in 7 until they lost their place at the pre-school. So you send them in and so does every other parent.
Unfortunately, there are knock-on effects to the NHS in all this. Due to the cost of childcare, childcare settings shape family spacing, so lots of three-year-olds (who are superspreaders on account of thinking hygiene is what you say in the morning if your key worker is called Jean) have very young siblings at home. Newborns and small babies are very susceptible to respiratory illnesses so that leads to more admissions in hospitals. And then there's the elderly relatives and immune-suppressed of all ages.
I think there is a lot to be said for seeing if we can cut the numbers of hospital admissions needed for complications of ordinary viruses. We're going to need the beds.