You Can start seedlings in a cold frame...but it depends on what kind of seedlings..

Cold season stuff (lettuce, spinach, onions, cilantro, etc) will germinate even in really cold weather if the ground does not freeze.
Hot stuff ( tomatoes, peppers, eggplants, beans,etc.) need a minimum soil temp of around 75 degrees or they will not germinate...

A lot perennial herbs actually germinate better if they have a period of cold followed by a period of warm. But if the soil temp.stays low the seeds can rot,or the seedlings will dampen off.

You can use a seedling heating mat to help keep the temp. of the soil up....but you need electricity for them.

Another technique would be to dig deep down below where you want to place the cold frame....and add fresh manure (cow or horse is best) and then put a foot of soil on top of that...the manure will generate heat that will be captured in the cold frame...

If you have enough room in the cold frame you can also place some jugs full of water, painted black to hold heat....or some black rock can work as well...

The most important thing will be trying to keep the soil temp fairly constant (especially on the hot season stuff and the perennials) I suggest starting with something easy like lettuce...and get a high/low thermometer to stick in there....

Happy planting
~SeedPlanter