Does anyone have any suggestions?
Well. A tough situation. The only remedy that I can imagine isn't going to be any less tough.
Right now your little ones think that their tree is "home."
You've got to get them thinking that somewhere else is "home."
I'm not sure, but I think you want the little chickens to live in that little house next to the big red house. I'm not sure that's a red house. However, if that red building is the main chicken house, then I would put my tree chickens in there with the rest of the flock.
I would want my tree chickens to be assimilated into the main flock because they're all going to have to live together in the run area in the white fence. So they've got to accept one another or your tree chickens will just end up back in the trees.
Here's what I would do. I am not recommending you do it.
So, during the day, I get a good new-ish ladder under the tree (as per the manufacturer's instructions-far be it for me to tell you how to use a ladder when I can barely stay on one myself). I go out after it's pitch black and get those tree chickens out of the tree. I do not go out before it's pitch black or they'll just move higher up in the tree when I try to catch them. One at a time. Sometimes I can get some help, but sometimes not. It's always easier with help. I try to come up with a plan before I go out there ... you know, and sort of a pep talk like "Remember to not let go no matter how much they flap." "Last resort, grab them gently by the legs and don't fall!" "Don't forget they will be noisy; that's normal." "It's better to catch them now because sleeping outside in 0 degrees temps can lead to death." I try to catch the male first if they are male and female (if possible). I do that just because the males are usually more apt to jump out of the tree than the females and then I end up having to wait until the next night to catch them. Gloves. Long sleeves. Eye protection. Etc. Headlamp turned sideways but still providing some light, but not shining in the chickens' eyes.
Then I personally would keep all of my chickens in that large house for about 10 days. I would move two feeders in there and two waterers. Plus I would prop a board in the corner or place a hay bale or something so that the new chickens could have a place to hide if they need it. The extra feeder and waterer is so that they can get some water and feed when the other chickens aren't looking. Some times the newbies jump down early in the morning and eat before the others get down or the newbies stay down at sundown eating and drinking before going up on the roosts. You should probably look up the threads on this site that describe how to introduce new chickens into the flock. It's not always easy. But it's not always difficult either. It just depends.
After about 10 days, the newbies "should" know where their new home is and will likely have made friends with the other chickens and will hopefully stay inside the white fence. However, I would try to make the white fence more chicken proof or I also might take it down because once a chicken sneaks outside the fence it can be difficult for them to get back in and that might lead to roosting in the tree again. Ten days, though, isn't always enough. And chickens are unpredictable. So who knows if that will work for you.
Hopefully, you or someone else has some additional or different ideas that might work. This chicken stuff ain't so easy. I had to go out and catch some the other night. And in Pennsylvania, where it looks like you are, gets very cold. Those chickens could get their little toes frostbit. That will be much more work and trouble (for them) to deal with than catching them now and getting them retrained on where home is. At least that's how I see it right now.
My chickens do okay with this kind of management, but you know your chickens and what they like and can deal with. So go with your gut on how to deal with this situation. Chickens are lovely, but it's not easy figuring them out.