I hope I've been able to understand this well.
This is a peculiar problem. It started when I tried to set up a w11 laptop, that was surplus to my needs, for my grandson. I created Microsoft account and an e mail address because he is a child I had to set myself up as his parent/guardian.
A Microsoft account is an "object" different from an email address, that is another "object". Did you create two new "objects" that did not exist previously (an MS account and an email address) or only the new email address? Does the new email address have a Microsoft domain? I'll call it "grandson @ outlook.com" for now, please correct me if the domain isn't Microsoft's (Gmail or Yahoo for example), there're important differences between the two cases.
However for other reasons the setup was not working well so I decided to reset the laptop to factory settings by reinstalling Windows instead. This was successful. Now however for some unknown reason it appears that my e mail has become my grandson's
Are you meaning in the reset laptop or in other computer, your own computer I'd assume from the context?
In the latter case it seems you have recycled your MS account as your grandson's one, instead of creating a new MS account for him. If you did this you cannot re-use your email address again for the same or other MS account. If its domain was Microsoft's its usability is compromised. This option is little probable or impossible unless you could somehow (and in error) change your account's personal identity (name, birth date,...) to tour grandson's.
In the former case you seem to have changed the laptop's MS account from your MS account to his MS account first. Some reset/reinstall procedures will not change the MS account that "owns" the computer back to you. In such case you're trying (unknowing it) something impossible and unnecessary: change the account personal identity from your grandson to yourself. But this would be good news actually: instead you should swap the accounts, setting yours as the one that owns the laptop. Even if you have changed your mind about giving a computer to your grandson, he will still retain the MS account and email address you have created for him, with or w/o computer to use it.
and initially I was severely restricted in the hours I could be on line. I managed to make my wife my "guardian" and was able to hours and on/off time to a fairly reasonable duration. My account is now known by my grandson's name.
This is just a workaround but not a solution.
I have tried every which way I can find to change my account back and although I can change the name back to me I am unable to change other settings such as my birth date so I am still dependent on my wife being my guardian. The Microsoft web help site has lots of options but I can't find one that corresponds to the problem I have. Is there anyone who can help me restore my account fully to what it was?
As said above, it depends essentially on if you created an actual new account for your grandson. Swapping the laptop's account would be the solution.