What temperature and light conditions do you have?

I tend to think the temperature has more to do with them not growing. I regularly have them come up as volunteers in my south facing flower bed. They germinate when it is quite cool and then just sit there until the weather warms. I have put a tunnel cold frame that I have over them at times if I want to keep them and then they start growing. If you can increase the heat a bit they might start growing. A hundred watt incandescent bulb will throw enough heat to give them a bit more heat.

Transplant when they have their second set of true leaves. Right now they only have their seed leaves.

They're in a un-heated greenhouse that gets sunlight for about 7 hours (on a good sunny day) and it stays around 10 degrees on a cloudy day but when sun breaks through it can reach up to 25 degrees Celsius.. When does the second set of leaves come through?

If it's 10 degrees on a cloudy day what is it at night I'll bet it is close to freezing or below. Your snaps won't do much growing with those temperatures.

Take some clear 2 liter pop bottles or any clear bottle and fill them almost full with water, Put the lids on and let them sit on a sunny shelf in your greenhouse. The water will get hot. Before night time arrange the bottles around the container with your seedlings. Wrap bottles and container in a sheet of plastic or cloth--don't let it touch your seedlings. You will have made a little warm coccon for your seedlings. Remove everything in the morning so you don't fry your plants and repeat every evening until the weather is warmer.

Your seedlings simply won't grow with the temperatures they are at now

I will agree with dowlinggram. When I grow snaps they are kept about 15C all the time......and they creep...ohhhh they creep.... until outside to explode.... after 8 weeks I have transplanted snaps that are under 3" tall but have great roots....

good luck