“一下儿”可以表示时间：如，等我一下儿；也可以表示次数：如“拍皮球，一下儿，两下儿”；还可以做形容词表示很快、迅速的意思：如：她一下儿就崩溃了。 “一会儿”大部分就用来表示时间或时间段：如，等我一会儿。。。或再过一会她们就来了。

the difference between "一下" and "一会儿" is just like the difference between "often" and "usual", though their meanings are opposite. "一下" can be considered as "a time" or "once for an action". it generally used as "verb + 一下", eg. "找一下王先生"(to find mr. wang). "一会儿" means "a while". but sometimes there is not much difference between "一下" and "一会" when they both mean "a while". for example, we can take "等我一下"(wait me for a moment) as the same as "等我一会". it's usually used as "verb + obj. + 一下/一会"

the difference between "一下" and "一会儿" is just like the difference between "often" and "usual", though their meanings are opposite. "一下" can be considered as "a time" or "once for an action". it generally used as "verb + 一下", eg. "找一下王先生"(to find mr. wang). "一会儿" means "a while". but sometimes there is not much difference between "一下" and "一会" when they both mean "a while". for example, we can take "等我一下"(wait me for a moment) as the same as "等我一会". it's usually used as "verb + obj. + 一下/一会"

the different between 一下儿 and 一会儿, i want to take a metaphor to explain. 下：you can imagine "drop,down,fall" if there is a drop rain,its light and soft; if sth down or fall ,the speed fast（time fly） then 一会儿=for a while. 一下as a drop；（more fast，happen in the twinkling of the eye） 一会as a line.（quick，fast）

倾向于动作时多用“一下儿”。 表于时间时，没有什么区别。 他一会儿就到，你再等一下儿吧。 他的头被碰了一下儿，真是不当心。 屏幕刚才闪了一下。