Your profile shows Bogota Columbia but from your photo you are not downtown. I don’t know what animals you have down there, but that type of terrain around here near cities teems with raccoons, possums, skunks, snakes, bobcat, foxes, coyotes, hawks, owls, and dogs. Raccoons will use storm sewers as highways to safely get around town in purely urban areas. I’d be surprised if you don’t have quite a few of your Columbian version of many of these predators.
Still just because predators are around doesn’t mean they are coming tonight to get your chicks. My parents raised chickens in a true rural setting with predators all around with less physical protection than you have and would usually go years without an attack. But when a fox or dog found them, it would have to be dealt with. That’s the thing with predators, they might not come around for years or they may come tonight. A good dog can help.
To your basic question, will that hold in chicks? I’m assuming you mean just hatched chicks, not adult birds. Probably. Some of that depends on how tight you build it. Chicks are pretty bad about squeezing into tight places and getting stuck. Under that down they have pretty small bodies. So that is one risk.
Something else you will find is that it doesn’t take long for chicks to be able to fly pretty well. I’ve seen 2-week-old chicks fly 60 cm vertical and a meter horizontal when Mamma told them to come to the roost. Just from watching them fly, they could have easily gone even further. They also like to perch. It seems to be an instinct for them to get on things up high and relax there. That top horizontal you are attaching the limbs to might look like an inviting place for chicks to try to perch. I’ve seen chicks perch on a 2x4 against a wall, which means they had 37 mm to land on up flush against a wall. It doesn’t always take a lot to attract them.
I don’t know at what age yours might be tempted by that horizontal or how big they will be, but I would not leave them a lot of room to get through up at that level. That dead hedge fencing idea could take care of these potential problems.
Good luck with it.