You should use a forward declaration of the protocol DataChildDelegate
prior to DataParentDelegate
so that the compiler can trust that it exists.
For example:
#import <Foundation/Foundation.h>
@protocol DataChildDelegate; /*Forward declaration of DataChildDelegate */
@protocol DataParentDelegate <NSObject>
@property(nonatomic) id<DataChildDelegate> firstChild;
@property(nonatomic) id<DataChildDelegate> lastChild;
@end
@protocol DataChildDelegate <NSObject>
@property(nonatomic) id<DataParentDelegate> parent;
@end