In most cases, they are not interchangeable. In my experience, "儿童" emphasizes on age, but "孩子" emphasizes on relationship.
When you are talking about a part of the population, "儿童" is more formal and can be used with other words to make phrases. Therefore, in the following occasions you only use "儿童":
"孩子" is informal when you talk about that population and is seldom used with other words. The only case I can think about is "孩子们". You cannot say "孩子医院". HOWEVER, it is the most commonly used word to indicate relationship, i.e. "我的孩子" for "my child(ren)". You cannot say "我的儿童".
By the way, for people in their 50s, they can call all people younger than (approximately) 25 "孩子们", but someone at his age of 20 is definitely not "儿童". That's why I say "孩子" emphasizes on relationship.
From a translation perspective, "儿童" can be translated into "kid", "infant", "child"; "孩子" mostly means "child". The meanings and usage of these two words are not strictly corresponding to those English words so be careful.
"少儿" is "teen", and usually only refers to that group of people (i.e. "teens"). It is kind of different...