I accidentally put my sons USB through the wash?

check with your laptop and its better to buy a new one..

A flash drive (thumb drive)?

They're cheap enough that you should plan on replacing it.

But, if there's critical data on it that you want to recover:

The odds are better than for any other electronic device going through the wash.

My inclination would be to open the case, and remove the circuit board and USB connector assembly. (Don't plan on putting it back together.) Dry it thoroughly, using the rice trick. Then, see if it works.

It's ruined, sorry.

Electronic devices and water do not mix.

It's almost surely not a good idea to plug it into a computer to see if it works either. (If you want to do so, try it on a computer that it is not yours, like at a library or an internet cafe. When their machine crashes, they may not be very happy about it.)

Instead of asking on Y!A, plug it into your f***ing computer and see if it works. If your files are accessible then it works, if not, then it's ruined. I hate questions like this...