How to delete all except for some specific folder under parent-folder

You can use find with a negation (at your own risk).

find all file and folders named "a" or "b":

find -name a -o -name b

find all files and folders name "a" or "b" in the current directory"

find -maxdepth 1 -name a -o -name b

find all files and folders not named "a" and not named "b" in current directory:

find -maxdepth 1 ! -name a ! -name b

also exclude current directory from result

find -maxdepth 1 ! -name a ! -name b ! -name .

now you can use rm to delete all founded elements:

find -maxdepth 1 ! -name a ! -name b ! -name . -exec rm -rv {} \;