I built a small safety space under the laying boxes in my coop. Mine were 6 weeks when I brought them out. (My family got attached to having them inside)
The littles use the top of the small run to eat and drink the bigs do not bother them up there.
Great idea - and I love the chick size pop door. You could've even raised the littles right in there if you wanted to. I was too attached to having the nest boxes at ground level though at one point I thought of building them up on a bench, and using the space under for chicks or feeders or storage.
I'd heard that adults were less likely to bother eating chick feed/water on a platform, so I turned the top of my chicken jungle gym into the feed and water area for the chicks. My hens do go up there, but are somewhat reluctant to do so, so it really does seem to help with providing the chicks some protection while they eat.