Luc is correct. When there is enough airflow to cause the plants to shake somewhat, the stems will naturally thicken to minimize the shaking, provided there is enough food to do so. The airflow is slowly increased as the plants get stronger, needing a stronger wind for the stems to thicken further. The problem here is that you want the plants to be at least standing up on their own before trying to encourage stem thickening. If they're having trouble standing up, more air will just blow them over & make it harder for them to stand back up.
When seedlings stretch towards the light, their stems stay about the same thickness while the size of the plants they support increases. They become more top-heavy and fall over more easily. More light, without more heat, will decrease stretching and the shorter plants will be better able to support themselves. Start work on the problem by increasing the light until they are strong enough for more airflow.
Another issue that could contribute to seedlings falling over is overwatering. Let the top half of the soil dry out completely before watering. You should also consider that males commonly grow faster & taller than females. They'll need at least weak nutrients when the three-bladed leaves start growing in. Maxicrop soluble seaweed is a great mild fertilizer for young ones.
