I usually get my tuba kids started playing 2nd partial by doing the following:
1. Drop your jaw until it hurts.
2. Touch the center of your lips together (the jaw will relax together a bit but the teeth will be out of the way). Keep the center of the lips touching - this is not a pucker-up face! Long and tall!
3. "Cough" (it is a short burst of air from an open throat that will force the lips to vibrate, albeit slowly)
For most kids, this will get them a quarter note Bb with no defined articulation. After a few attempts, they get the hang of how slow and open the lips vibrate and then can transition to a more natural exhalation.