Obviously it's Milk Cereal first for a number of reasons:
> You make a mess if the milk overflows.
> It's hard to tell how much cereal you use if you don't put it in first.
> I put in enough milk to almost cover the cereal, so without putting in the milk second, you can't tell.
> If you put in too much/the wrong cereal, then you can scoop it back out. That is, unless you put the milk in first... xD
I know people that put in juice rather than cereal, but either way - all logic and the laws of nature clearly point to the fact that you should put the cereal in first.