I would not let them run free , as you said there’s dogs near by one mouthful and you’ve lost 4 chicks .
Get a dog crate take the plastic tray out so they’re on the grass. A fence won’t help because they will be flying over it soon.
With 36 it could go bad very fast! Hope for the best but plan for the Worst
what size dog crate will hold 36 half grown chickens ??