Yes, KINDEST is a Scrabble word!

Letter values in Scrabble US: K 5, I 1, N 1, D 2, E 1, S 1, T 1.

Is it a valid word?

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

having or showing a tender and considerate and helpful nature; used especially of persons and their behavior
agreeable, conducive to comfort
tolerant and forgiving under provocation