I'm in the same exact situation. My first group of birds are 15 weeks old, my second are 10 weeks old. At present I have a total of 9 chickens. We are rehoming 3 cockerels

. It has been in the 90s here and very hot. I usually give them scratch in the morning (wild bird feed, not too much crack corn, which builds up heat internally), to keep the first batch away from the young batch of chicks while I put them in their cage in the run. I usually have frozen berries and cold watermelon for when it gets really hot in the day and cucumbers, also broccoli, brussel sprouts & kale leaves, all straight from my gardens. We have an enclosed run, my German Sheperd is not chicken friendly, no free range here. I also give them a small amount of black fly larva before bedtime. They still eat a good bit of chick starter/grow feed and search for bugs. I spoke with my uncle (he kept about 200 chickens, show birds, pets and livestock), this is what he told me, make sure they are still eating the grower chick feed, leafy greens and veggies are good, limit spinach to only small amounts, fruit is a treat, so should be in small quantities (but frozen on hot days, is great for cooling them down0. Try to stay away from anything processed, but if you do indulge, make it very far and few, especially in hot months. Crack corn is good for cool weather will help build body heat. I have never given my birds oatmeal. I may try that in the fall. Anyway, he told me to watch the chicks, see how they respond (foraging, healthy, energy level wise). I wish you much success with your flock!!