Put the lemon juice, mayonnaise, yogurt, tomato ketchup and Worcestershire sauce, if using, into a small jug and mix together well. Add freshly ground black pepper to taste.
Halve the avocados, remove the stones, peel and thinly slice. Slice the mangoes in half, cutting either side of the stone, peel and cut the flesh into long thin strips. Set some avocado and mango strips aside on plates for toddlers.
Tear the little gem lettuces into small pieces and scatter over 6 small plates. Top with the avocado and mango strips.
Spoon the king prawns on top of the avocado and mango, and dollop a little of the delicious dressing on the side. Sprinkle each with a pinch of paprika, if you like.