This is a great phrase to use for reference when it comes to learning the meaning and use of the grammar particle 了 (le).
Notice that the situation or state has changed here. A kid is sitting on the toilet, and has finished doing what one does there - then he shouts to his parents "I'm done now!" requesting to have his bottom wiped clean.
He wasn't done a moment ago though, so the state has changed. He wasn't done, but now he is - so it is appropriate to add the small 了 (le) grammar particle.
It isn't very easy to learn this grammar item, but here is a good phrase for reference.
By the way, enjoy the phrase :)