Can you just move the little red coop into the indoor run, close the bottom part with wire, and fix some space inside for them to roost or even just huddle off the ground?
They'd have (1) 24/7 protection from weather and predators, (2) a see-don't-touch space where they'll be with the older hens all day but safe from them, (3) plenty of space for three up to about 8-ish weeks of age.
After a week or two of this living inside the hens' space, I'd open up several holes in the wire enclosing the bottom of that red coop, just big enough for the littles to get through, but too small for the hens to follow. And add a bunch of obstructions and obstacles in both run areas - turned over chairs, logs, stacks of blocks, planters, etc. Wood pallets are GREAT; I put them up on bricks or blocks to raise off the ground, and the littles can run underneath for safety.
Once they're integrated this way, you can remove the little red coop. But you'll have to teach them to roost in the blue coop. Not a big deal at all. Just pick them up and put them on a roost inside. After a few nights of this, they'll go in on their own.