The search would be:
not tags:childrens
You can build searches using the Tag Browser.
first click is a Green Plus, Second click makes it red minus
Here is an example of a multi criteria (saved) search
Code:
((formats:false and not #master:"LNK") or #usertags:"missing")
In this case
No formats <<wishlist entry only
The #master is
NOT "LNK" <<< Just a cross ref to another book. Omnibus containing...
#usertags has
missing as part of the tag
Saved searches can be a basis for a VL eg PGlib for children
Saved searches can further refine what is shown
In my case I do not keep read books on my device, I log the date in {#read} in calibre. So my To Read VL only shows those
(false in some cases may include Not set, thus Not True

)