すえっこが いちばん かわいい。
すえっこ(末っ子)= one’s youngest child
Does this mean the youngest child looks the prettiest?
かわいい is usually translated as “cute” but it also has a meaning of “loved” or “lovable.”
So すえっこが いちばん かわいい means more like: the youngest child gets the most love.
かわいい in this meaning is used in a proverb:
かわいい 子には 旅(たび)を させよ。
The child you love should be made to experience (the dangers of) traveling. [Spare the rod and spoil the child.]