Personal opinion: I wouldn't do it myself or let my kids do it. Why? It reinforces the idea that you can just take a pill for whatever ails you instead of taking responsibility and doing what's right for you health so that you don't need pills. In this case, it would be to eat right and avoid the carbier foods. Kids are already bombarded with commercials and ads for the latest and greatest drug which send many adults to their doctor's office demanding the latest and greatest med even when they don't know what it's for or if they even need it. I personally don't want to reinforce that idea by encouraging my kids to take over the counter stuff when avoding the starch that they supposedly block in the first place would be a better option.
Most carb cutters also only work on starches, not sugars and have not been tested for safety in children. Given that they have a tendency to block absorption of vitamins and minerals along with the carbs, I'd especially avoid letting a growing child take them.

Just my opinion, though.