一下 means "in a short while", for example:等一下 it means "wait a minute".
儿 it is a noun,means "son"or "somebody's kid".While in China especially the northern area, such as Beijing, they will add 儿 in the end of a word to show they affection. But sometimes add a 儿 in the end of a word, it will change the part of speech. In formal language environment, usually we do not add 儿 in the end of a word.