I don't know what it's like to have a child skip many grades, but no parent or child should be apologetic for who they are, whether it's an achievement or a setback. It's too bad so many people are rude and small minded.
This follows people into adulthood. Being very successful at what you do should not need any apologies, but it does. My DW is the same down to earth person she has always been, but she is not forthcoming with her role any more, due to peceptions about what senior execs do. When she was a secretary, then she was one of the girls and no one felt threatened, but not any more.
Intelligence has a qualitative aspect to it that impacts most of one's life. Being bright, being scary smart, is MUCH more of an impact on one's life than being able to throw a ball well. If someone can throw a ball, that usually does not translate into one's job performance or professional occupation. But having an IQ of 135, does - it enables things in many more aras of human endeavors.
Thinking and perceiving are something that everyone does and its tied up with the notion of self in everyone. Ball throwing is not tied up in most people's self. Someone who can outthink and out perceive you means that they can outperform you on something that is intrinsic to your identity. Its easy to see why that is threatening.
Another identity people have is their kids. We all know this.
The isolation parents feel comes from not being able to share and discuss, but also from that threat to others identity such a child represents. Superior intelligence is disruptive in many, many ways and not being able to understand why a child is so much further ahead - and knowing that child has more to offer than your child just adds another threat to that identity.
A smart adult is a threat, but a smart kid with a supportive parent is a much deeper one.
I also think that due to human history where one group from the outside has always displace another - there is a deep seated fear of others that is intrinsic to human identity. Smartness may tap into this as well.
