10 days is too young to remove access to a heat source for long.
I have chicks the same age brooding on my back porch in Florida. The other day the ambient afternoon temp was 95 degrees, so I turned their heat off and they were perfectly content. 
The following day (yesterday) the high was only 90 degrees and they got very upset when I tried turning it off. Just to give you an idea of how small a range they are comfortable in.
If you have a larger brooder (yours looks too small, sorry) then they can self-regulate their temperature by going to the cool end and back to the warm, but in a small area there is no cool end.