Up until 7 or so, it's really a waste to take kids to a magic show, because everything is magic to them. That's why they can so easily believe in Santa Claus. So if they were told their dad was Superman, they might assume that everyone's dad had super powers, and it is something that happens when you grow up. Then, when they learn everyone is not super, they might refuse to believe it about their dad too. I had actually begun to outline a vignette with that plot. (Not that I ever get around to actually finishing any fics.)
There is also a change in thinking and understanding that takes place at about age 14. I remember once reading guidelines for allowing a young person's testimony in court, and I think it was assumed that anyone over 14 could be considered a competent witness; anyone younger might be competent, but it was not assumed automatically. I have to admit that I read this over 10 years ago and my recollection is fuzzy, but my recent experience with kids tells me that high schoolers do understand things a lot better than middle schoolers.
So 14 might be a good age, except for the fact that Clark's powers began to appear at puberty, and girls mature sooner than boys. Some girls may show the first signs of puberty as early as age 9! So the question is whether any inherited powers will begin to appear before the child is mentally prepared to keep the secret. With luck, the two will converge at about 12 or 13.
Just my 2 cents.