I've never had a problem with a chick figuring it out. They should be able to feel the heat and that will draw them. Plus, going under something is a natural instinct. Just make sure they aren't plunked into darkness by the flick of a switch at night. Chickens can't see well at all in the dark.
I'm late to this conversation but wanted to point out that EcoGlow has a minimum ambient temperature of 50F. It's meant to be used in an already heated space (like a human house) but also suits mild climates.
It wasn't enough heat for my outdoor brooding purposes so I went with the DIY mama...