I wasn't speaking to a child. I spoke about the content of the issue. What form to present it in, and which content the child would be interested to hear, is not my affair; it depends on the details of their situation and personalities. I cannot hope to comment accurately about that. Nor can you.
In the future I would strongly encourage you to leave out remarks such as, "You should understand more about child development." I don't mind if you state that I'm wrong, for clarity, but you don't know anything about how many years I've spent learning about child development, or how much I know about it, so it's not wise to comment on that.
PS I upmodded you back to 1. I don't think you intended to be hostile.
Again, no offense, but the set of people who understand child development and the set of people who would say what you said do not intersect. So I can infer that you do not know child development.
There are two questions here: (a) what's the actual value of this idea, and (b) what's the right thing to say to a child who has come up with an idea like this, in order to give them the correct amount of encouragement without over-praising them?
As the cold-hearted over-educated curmudgeons of the internet, we're allowed to answer part (s) without worrying about part (b). The kid's dad has to worry about part (b) for himself, but I hope the kid's dad has enough perspective to realize the correct answer to part (a) is that it's worthless.
I don't really think any reasonable reading suggests the kids dad was asking question a. Do you?
It's a useful number because it gets a kid playing with math. Encourage them to teach themselves more about it, to play with it, but, for Christ's sake, do not give the child a terse answer more appropriate for a college student then a damn 6 year old.
Your attitude is at the root of why the entire edifice of mathematics education to children is completely soul numbing.