Investigate all laughter.
You will think they are playfully enjoying each other's company in their rooms. But once you decide.. "you know, that is some pretty ferocious laughter... what is so funny?" You will head down the hall towards their room. Only to notice the bathroom door is open. And the ferocious laughter is coming from there. And to your horror you will see the three year old, with his underwear around his ankles from having just peed in the toilet, splashing the water back and forth with his brother. The two year old, you will see, is not only splashing but then sticking his hand in his mouth.
People, you can't make this stuff up. Which is why you will always want to investigate all laughter.