It may be that success in such areas is irrelevant to the kid or to the parents or both. I did a *lot* of competitive sports, science fair, spelling bees, etc., in school and I enjoyed them immensely. I'm glad for the opportunity and hate to end that option for my kids.
I've got a very different perspective here, but the difference might be attributed to personality. It seems like I entered and won every coloring, drawing, writing, spelling and science competition that came along while I was in elementary school (I even had poems published in the newspaper and dioramas displayed on public access tv!), and in the end it was a very damaging experience for me. Being at the top became my identity and by junior high I was avoiding any competition I couldn't win for fear of being revealed as a fraud. This included any and all forms of sport. By high school, I was a poor performer in competitive activities that I should have been good at, like debate, because I was so stressed out about appearing smart. Carol Dweck has written about the difference between documenting intelligence and developing it, and too often, competitions were a way for me to do the former--to "prove" how smart I was.
Ultimately, I think the biggest problem was that the competitions I entered in elementary school were all rigged in my favor. Until high school, I wasn't competing against anyone close to an intellectual peer (my mom turned down a second grade grade skip), so I never learned how to lose. For this reason, I let my uncoordinated DS4 participate in sports with kids his age (he still has soccer with his friends from preschool), but I would not want him entering any kind of academic or art contest against kids his age. It wouldn't be fair to anyone, including my son.
It'll certainly be interesting - he's already the smallest in his group, so he'll be tiny compared to the 7 and 8 year olds - but he is SO up for the challenge - yeah 