If you were to read down this page, and keep going onto the next and the one after that, you will see your question asked here almost every day. It's a very,very common question.
Way back when I just had two adult hens, I put my very first chicks in the pen with them, not knowing what to expect since I was a chicken beginner like you. The first thing the big hens did was to eat the four-week old chicks' food. That's how my idea for a "panic room" was born.
I rigged up a very sloppy, quick chick pen, maybe 2' x 4', in a corner of the pen, and I cut a 5" x 7" hole for the chicks to be able to go in and keep the hens out. It worked great!
As it turned out, those hens were gentle with the three little chicks, but sometimes hens will be pretty rough on chicks, and if the pen is too small, the chicks will get trapped with nowhere to escape. Injuries and death can occur. This is why most folks wait until the chicks have gotten big before putting them with the adults.
Since then, I've perfected the "panic room", and have used it for ten batches of chicks, introducing the chicks around five weeks of age. They quickly learn to run into the panic room when they get chased, and their food and water is always available to them without having to be afraid of being chased off by the big girls.
It's always a good idea to let the chicks and adults get to know one another before letting them mingle. After a week or so, then open up the small openings so the chicks can explore the run.
Later, when they are successfully integrated, then you can try moving them into the coop with the adults.
