Welcome to BYC. Glad you decided join our flock. I'm a big fan of coarse sand (or fine, smooth pea gravel) for the floor of both coops and runs for the following reasons:
Coarse sand and desiccates droppings, drains well and does not retain moisture, does not decay or degrade inside the coop, is inexpensive, is eco-friendly (it can be removed from the coop/run, washed dried, and re-used), is a natural grit (do not use fine beach sand as it can sometimes impact chicks' crops), is easy to clean up (just some scooping every few days and an annual change), keeps chickens feet cleaned and nails manicured (cleaner feet means cleaner eggs, especially in rainy conditions), provides a natural dust bath, no decomposition is required in the compost pile (it's a great soil amendment to compost), it keeps coops cooler in the summer, and is an outstanding choice in winter as it evaporates moisture rapidly (reducing the risk of frostbite),
Raising chickens is a wonderful experience for children. My granddaughter (pictured in my avatar) loves our chickens. Please feel free to ask any other questions you may have. We are here to help in any way we can. Good luck in getting chickens.