Made these for my nephew last Thursday.
2-3 teaspoons of milk
Add few drops of cream cheese rather than pouring the whole package.
Add more butter/powdered sugar to thicken the mixture. Add milk to make it thin.
Adding yogurt can make a great difference to the frosting(you'll love it), but use it only if adults are to eat the cupcakes because kids prefer sweeter frostings rather than a tangy one.