“Adult herd immunity” is not herd immunity because the herd necessarily includes children.
Herd immunity is when such a high proportion of the population are immune that it prevents the spread of infection, so that even the non-immune are protected. That can’t happen at present; when the virus can spread through some group such as children who are mixed in with everyone else, it can reach anyone and everyone.
“Adult herd immunity” is therefore a contradiction in terms.