5 day old big stems little leaves? Why?

Light distance is 99% of times the culprit.... Also keep air moving or they might fall over and add a little more soil to the base to give it stability.

Insufficient light causes them to stretch like this. Either support the plants or transplant and bury the stem down half way and give them more light. Cheers

Not strong enough light is causing the stretching. You will either need to put the light closer or get additional lighting. Try writing the names on the pots before planting the seeds to avoid mix ups in the future. When the seedlings are ready for transplanting, you can bury them deeper so the long stems are further underground. This will be ok for them and in time, roots will grow from the buried portion of the stems. You may need to provide some support to these seedlings until they are ready to transplant and take care when watering them, as this is the time when they are most easily knocked over. Hope this helps,... Organoman.