what is the difference between 小 and 少？

In general,小 is related with size. 少 is related with quantity.
For examle: 我要小杯的 這件衣服很小
我要少冰的 我的錢很少

小：pequeño/a 这颗苹果很小
少： poco/a 工资太少了，根本没办法去旅游。

in English both mean little cause little has more than one meaning in English,
while in Chinese we use two characters to mean two meanings of light.

两者当然有区别。小是小，小的反义词是大。少是少，少的反义词是多 和 老。
可以说小孩，但不能说少孩。 可以说少年，但不能说小年。

小 Pequeño/a: Ejemplo: 苹果比西瓜小。
少 poco/a: Ejemplo: 你穿得太少了。

I want to explain the two word different in figures and abstract, formal or informal .for example ,we know that the level in army, they have 少将 少尉 少校. deep down, these three 少 level really are the lowest level，cause in this situation , need more serious and formal . this is for your reference only.

小means small,smaller,the antonym of 小 is 大(big, bigger). 少 means less then,youner, the antonym is 多(more then), 老(older)

小 is small, 少is less。

小 is for size and age.
猫很小。The cat is small.

少 is for number (amount, quantity)
猫吃得很少。 The cat eats very little.

first one is xiao meaning small.. da he xiao ( big and small)
2nd one is shao meaning little in quantity duo he shao (many and little) duoshao together means how much pointing at quantity..