What is the splash text?
When Minecraft first launches, the flashing yellow text that appears across the logo is called the splash text. The exact text which appears is chosen randomly from a predefined list.
Customizing the text is a cool way to surprise your kids on their birthday, or just generally add fun to the game. And even though most of the phrases are harmless, some parents aren't happy with one or two of them ('Down with O.P.P' seems to get a few mentions).
How do you change the splash text?
The phrases live in a simple text file called splashes.txt within the .jar folder (if you need help finding the folder, check the How To Find Your Game Files article). Simply edit the file to add your own splash message, delete one you don't like or change the wording.
Note that the message which is displayed is chosen randomly, so if you add your text to the list there's no guarantee it will show up the next time you or your kids log in. So if it's important that it be loaded the next time you play, take a backup copy of the file and then delete all other splash text phrases except yours from the file.