Nittos come in 34-35cm widths.
As a fellow parent of a young track rider, I would steer away from the end add ons. I've seen way too many simple crashes happen due to youth to say all those sharper edges would be safe. You wouldn't get onto our track with them on. I've seen a whole lot of juniors grow up over the years and none were terribly troubled by standard width bars and none seemed to be disadvantaged by them either. Kids are wildly adaptable