as above said, they aren't interchangable. 少し（a little, a few） is an adverb , which modify verb, describe degree; and 少ない(little, few) is an adjective, which modify noun. as above example.

They are nnot interchangable. "sukosi" = a little i.e I only speak a a little = sukoshi dake hanaseru or I only want a little = sukoshi dake de ii yo. 'sukunai' = there isn't much or many I don't have much money = okane sukunai yo or..there aren't many people = hito ga sukunai.

It is difficult to use 少し instead of 少ない. If it is changed by the scene a little, it is possible to use it. Case 1. 水が少ない (no words after 少ない) 水が少し instead of 水が少ない is a little odd though 水が少しだ is safe. Case 2. 少ない水 (some noun after 少ない) 少し水 instead of 少ない水 is odd though 少しの水 is safe. (Strictly,there is a very slight difference between 少ない水 and 少しの水. Unfortunately, I cannot explain it by English.) (maybe.. "少ない水"="water which is a little" , "少しの水"="a little amount of water")