I hope someone can help as I am new to germinating plants from seed and my problem is that they are shooting skywards at a great rate!

My main problem seems to be the tomatoes but the marigolds and petunias seem to be following the same pattern. I germinated them in the front bedroom which is the lightest place and have faithfully kept it at around 20 degrees. The plants came through about a week ago and the tomatoes are already over 2 inches tall, but of course with painfully thin stems.
Should I try to repot them and cover some of the stem or should I just leave alone? Should they stay indoors or would an unheated greenhouse be better?

The days are warm here now,in France, but nights are still only just above freezing.
Any advice would really be appreciaated as I don't want to fall at the first hurdle.
Thanks in advance
Sue