LED's don't produce all that much heat downwards, most of the heat is radiated upwards. In order for your LED's to raise the temps of the pots or your soil several degrees they will have to be quite close and being that close will most likely damage the seedlings once they break soil due to the light intensity.
You are better off using an HID (MH/HPS), a cheap halogen/incandescent bulb, or even a reptile heat bulb. These will provide significantly more heat without having a dangerous level of light intensity.