Hello, my name is Marcus.
As you can imagine this has become quite an issue.
Firstly as you have no order in place then it's a matter for you you too agree.
I appreciate that may be somewhat difficult and ordinarily if there is a dispute an application can be made to the court.
However in light of the circumstances it is likely to take the court quite some time to process any such application.
Therefore strictly speaking you can decide to isolate the children and there is little father can do about it at this time.
Ultimately though an application can be made to the court by father who may take a view that father can also quite easily isolate the children whilst in his care.
Obviously the approach of the court is unknown.
I also believe it will depend on father's circumstances. If for instance he is still working say for the NHS the court are likely to be supportive of you. If however he is self isolating at home then there is little risk to the children and as such the court would order the present arrangements to continue.
But in short in law there is nothing stopping you from taking the lead.