You can feed it daily. I would only give 1/4 tsp since yours are not used to it. If you notice soft stools back off from it and push the loose Timothy hay.
I personally do not feed "any" supplement to a chin under 6 months. A 10 week old kit needs its nutrients from a quality pellet and lots of hay. For that young of a chinchilla, I would only offer prepared wood sticks as a treat.