Sunflowers grow so fast that the only real way to get good strong stems is to provide them with tons of supplemental light very close to the seedlings as they grow. That can be difficult to do without expensive light set-ups as the common fluorescent shop lights most of us use just don't cut it. T-5 fixtures almost touching them and with a fan to reduce the heat work best but I have found that even then they can get a bit leggy.

They need intense light like metal halide if grown indoors to prevent them from weak stem development. Which is why direct seeding them outside it recommended.

Dave