People are romanticizing smart children and being smart since childhood, making it sound better than it actually is. I was not a smart child, but there are few flaws about being smart since childhood. People make it sound and look like if you're a smart child, you have better childhood than other children, which doesn't sound true to me.
Most of children who are smarter than an usual child often end up feeling empty or not happy in their childhood or when they grow up, and knowing too much kind of takes your childhood away, making you feel different from other children and feeling like you don't fit in.
I do not like the word 'dumb' so, i'd say, children who are careless and unaware of life are the happiest. Most smart children have grew up too quickly, and didn't enjoy their childhood. I often feel a bit sorry for smart children, i congratulate them, but i also feel like being smart can ruin childhoods.
Note: I am talking about children with high IQ and children who know something they should know when they grow up, google search Kim Ung Yong for a good example. Nothing wrong with teaching a child about life, but they should be careful with what they are learning, children who already know algebra and difficult equations feel a bit miserable. But children who know how to count from 1 to 10, who know names of animals and what sounds they make, who can speak their native language and know how to speak their language correctly without too much mistakes are often happy, because they are learning the basics of life which aren't too complicated.

This sounded better in my head so, sorry if this post is boring. This is just my opinion, but there can be happy people who were smart since childhood or children who are smart AND happy so, feel free to disprove me.