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.

一般用在口语里，表示时间不长，例如：等我一下儿、我想出去一下儿、让我考虑一下儿、钱一下儿就用完了。

一会儿，时间很短