I don’t understand why people think the parents should isolate too, or not attend the wedding. Unless of course they have not had chickenpox. In which case they could themselves be affected, and infect others. If you get a very mild dose of chickenpox, you can get it again, but I think this is pretty unusual.
There is a common fallacy that people can get shingles from being exposed to chickenpox, but this is no longer thought to be the case. Anyone who has ever had chickenpox carries the virus in their body and it can reactivate as shingles. And this can happen more than once. That’s a separate issue.
The child clearly should be at the wedding if he is still infectious. If they is someone who has had chickenpox, and is willing and able to look after him, I don’t see why the parents shouldn’t go to the wedding?