Chickens and chicks like to scratch at the ground to find food. They will scratch or knock feed out of the container so that it lands on the floor to eat. If they have plenty on the ground don't jump to filling the feeder right away, if you keep filling it, they will keep taking it out. They get a little better as they grow up but they still do this. Layer pellets should be fed when they are laying around 20-24 weeks depending if you have the early layers otherwise if you have a breed that lays later, I would wait to prevent health issues.
I do a hanging feeder in the brooder to help a little, I also don't overly fill with pine shavings so that they can scratch the feed up. It all depends on what you are using for a brooder, if it doesn't have a floor, the idea of a cookie sheet is brilliant!!