We are discussing average kids, who can achieve average B+ in school. What determines their success in chess? It's not their smartness, but their persistence.
Chess growth is not a sprint or couple of sprints. It's more like jogging a marathon, with a few sprints in between. To grow above 95%, or even 99% of all players who have played chess, it takes about 5 to 10 years, or approximately 5,000 hours (just my guess, no scientific proof.) During these long growth years, your child may have some stagnant periods (plateau), or back tracks. S/he may get so discouraged by a series of bad tournament results that s/he wants to quit. S/he may feel tired and no energy to study chess at all. S/he may feel peer pressure and wants to merge into the 99% non-achievers. At all these moments, it's important for your encouragement and support to continue with one more step. S/he have to survived his/her low points, then s/he can climb out of the valleys and continue his/her growth.
At the bottom of valleys, it's very easy to call a stop, then we lose him/her forever.
Checking all those kids who started around the same time as my daughter, almost 99% of them stopped somewhere in last 10 years, some never really started, some stopped around 1200, some stopped around 1500, and a few stopped around 1800. Some boys were very smart, and did much better than my daughter, but nevertheless, they stopped. If they quit, I could not find anyone who actually came back. It's not that they are not smart enough. The simple reason is that they can't stay in the game. Persistence is the key factor in chess success, and in any endeavor in life. One more step, one more tournament, or one more game, s/he may regain his/her interest and confidence and get back on track again.