Seedling that do that are normally stretched which means a lack of adequate lighting. One thing that is good to do, besides giving your plants adequate lighting, is to leave a fan blowing on them at all times. In nature the wind causes trees and bushes and plants to sway back and forth and that motion is what causes the trunks/stems to thicken/grow thicker and wider. Without such movement plant stems will likely remain thin, or at least be thinner than they otherwise would be.
　
If it seems obvious that your stems are weak enough that a plant or most or all the plants may bend over completely you can gently dig them out of the soil. Being sure to take more then enough soil to get all of the young roots, and then replant them deeper. Place then deep enough so there is roughly one quarter of an inch between the top of the soil and the bottom leaves. The part of the stem that used to be above the soil will push out roots so your root system will expand/grow larger faster. If you do the replanting deeper thing be sure to make sure your plants receive adequate light and let a fan blow on them or else they will lonely repeat the same thing and grow taller with thinner stems.
