wow yes they are rather leggy. They need good bright light to slow down the tall growth.

I haven't started my cosmos off yet. How long have they been growing. You may need to start fresh seed if they 'collapse'.

I would suggest that you may have started to grow them a little early. Sometimes it's better to wait and even grow later so that they grow without a check in growth. Might work by leaving them in natural light by day and when they produce their real leaves, transplant them by pricking them out and planting them deeper.

Many thanks they are only in 4 days