I'm pretty sure the accusative is used with sehen as in "ich sehe dich". So why do we use the dative form for the plural definite article in this sentence? Am I missing something?
As an addition to RHa's answer please note the following:
I'm pretty sure the accusative is used with sehen ...
This is correct however there is also the possibility to use the word "sehen" with a direction instead of an object:
Ich sehe nach unten. (I look downwards.)
Ich sehe in den Spiegel. (I look in the mirror.)
The expression "jemandem ins Gesicht sehen" is formed of this variant of "sehen": "Ins Gesicht" is a direction, not an object.
The second thing you should note is that in German language many expressions are formed by adding a dative to another expression:
Etwas wegnehmen <-> Jemandem etwas wegnehmen
Die Haare waschen <-> Jemandem/Sich die Haare waschen
Ins Gesicht sehen <-> Jemandem ins Gesicht sehen