The foldy stool.

My kids are just a little too short to reach the sinks at most hotels and camp grounds, not to mention all of the rest stops in between. I found a little plastic stool that folds completely flat at a hardware store. It cost about $12, and I bring it along when we travel, sometimes in my suitcase and sometimes in the car. It was especially helpful at camp last week because it doubled as a chair. In hotels, I just set it up in the bathroom and then I don't have to help with the teeth brushing and the hand washing.