Tongue twisters are renowned for being tricky, getting tongue tied is a likely side effect.
Perhaps start off with an easy one for a pronunciation work-out, how about 'I scream, you scream, we all scream for ice-cream'!
Note for Mortals:
Soft Vanilla Flavour Marshmallows
Ingredients: Glucose-Fructose Syrup, Sugar, Water, Gelatine (pork), Anti Caking Agent: Maize Starch; Flavouring, Colour: E100, E120, E131.
May contain traces of nuts and peanuts.