Yes, CHEERIO is a Scrabble word!

Letter values in Scrabble US: C 3, H 4, E 1, E 1, R 1, I 1, O 1.

Is it a valid word?

CHEERIO is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

Also: auf wiedersehen, au revoir, bye-bye, good-by, good-bye, good day, so long.

Not playable in Scrabble US: ARRIVEDERCI.