It's not helpful and it's causing me a lot of distress
It should be causing you distress.
I am concerned you are minimising what he is doing, OP. You quite clearly know it is wrong as you are aware it would cause safeguarding concerns.
I feel sorry for the vulnerable children.
When your DH injures someone at work because he is high, it WILL be your fault as YOU have enabled him.
Lots of great suggestions on here which you are not acknowledging.
There will be a locally commissioned service who support families of addicts.
Yes - they will report the risk. BECAUSE IT IS A RISK.
And you know it and are more concerned about your DH than the risk to the children and his patients.
What he is doing is illegal and you are enabling.
Step up and do the right thing before someone around you does it for you and then YOU LOSE THE CHILDREN. Your nanny is not stupid and neither are others around you. It is just a matter of time.
Also, you are in hospital, so he is around the children without you when he is high. Great....
I am sympathetic with your predicament to a point, OP, so go and get help before it is done unto you.