The first thing that popped into my head is a "chubby bunny" challenge -- if you're not familiar with it, it amounts to filling your cheeks with as many marshmallows as you can reasonably fit, then you try to say "chubby bunny" while hopefully not spewing marshmallows everywhere.
The "BB" sounds make it hard to keep the marshmallows in your mouth - maybe using a "P" sound in the phrase would be equally challenging? Trying to say something like "Peter Piper Picked a Peck of Pickled Peppers" with a mouthful could be hilarious. Maybe a mouthful of water instead of marshmallows?
That's a maybe, it does work for all ages.