The legislation is here. www.legislation.gov.uk/uksi/2020/1045
To paraphrase:
"Self-isolate" means a person must remain in their home or the home of a friend or family member where that person is a child (inter alia).
They may leave their home to move to a different place specified (i.e. their home or the home of friend or family), where it becomes impracticable to remain at the address at which they are (amongst various other circumstances).
Arguably, it's impractical for a child to remain at their home if their parents are unable to provide childcare because they need to work and the child can therefore move to the house of a friend or family. The government may well disagree with that interpretation but I'm not sure a court would. I don't think bubbles are particularly relevant here.
What is interesting is the circumstances when an offence is actually committed though. These are much more limited than you might think.
A person who contravenes the self-isolation requirement "without reasonable excuse" commits an offence. In my view, a reasonable excuse would be to obtain childcare for your child so that you can work (particularly if your job or income is at risk if you don't). A reasonable excuse doesn't have to be one of the specified circumstances where you are permitted to leave home. It could be something completely different.
Also, in addition:
"A person who contravenes a requirement to self-isolate... without reasonable excuse and in doing so—
(a) has reason to believe they will come into close contact with another person or group;
(b) does then come into close contact with another person or group;
(c) is reckless as to the consequences of that close contact for the health of that other person or group;
commits an offence."
Are you being reckless by sending a child who probably doesn't have covid to fully vaccinated grandparents particularly while regularly testing child? I think not.
(The part of the legislation which sets out the offences relating to self-isolation is badly drafted and would likely to interpreted in favour of the defendant for that reason.).
My view is that in the highly unlikely circumstance that the police fined you for this, there is no way the CPS would pursue it on appeal or that a court could find that it is a criminal offence. There are too many get outs in the legislation.
Most covid fines issued have been overturned for this sort of reason.
Also, I cannot actually find any limits in the legislation on having visitors while self-isolating. It may be buried there somewhere but not obvious on a quick glance.
So my opinion is that you won't be committing an offence by doing what you want to do.
I'm not personally too bothered about the what the law is anymore and will get grandparents over (or anyone else who offers) to look after my child if he has to self-isolate again. I'm not going to risk losing my career or thousands of pounds over this.