bye–bye

bye–bye /ˈbaɪˌbaɪ/ interjection
bye–bye
/ˈbaɪˌbaɪ/
interjection
Britannica Dictionary definition of BYE–BYE
informal
: 1goodbye — often used by children or when speaking to children

go bye-bye

informal
1 
or British go bye-byes : to go to bed — used when speaking to children
2 
US : to go away — used in imitation of children's speech