Maybe the simplest way to implement Key-Value Observing compliant collection.

KVOCollection-Example.m

// This triggers correct KVO insertion notification:
[parent.mutableChildren addObject:@"Child"];

KVOCollection.h

// Key "children" is fully KVO compliant:
@property (atomic, readwrite, copy)  NSArray *children;
@property (atomic, readonly, strong) NSMutableArray *mutableChildren;

KVOCollection.m

@property (atomic, readonly, strong) NSMutableArray *internalChildren;

- (instancetype)init {
    self = [super init];
    if (self) {
        self->_internalChildren = [[NSMutableArray alloc] init];
    }
    return self;
}

- (NSArray *)children {
    return [self->_internalChildren copy];
}

- (void)setChildren:(NSArray *)children {
    [self ->_internalChildren setArray:children];
}

// Forwards collection changes from internal to public `children`.
+ (NSSet *)keyPathsForValuesAffectingChildren {
    return [NSSet setWithObject:@"internalChildren"];
}

// Mutable proxy works with internal collection that has NO setter. This is important!
- (NSMutableArray *)mutableChildren {
    return [self mutableArrayValueForKey:@"internalChildren"];
}

In case you don't need -setChildren: you could do this without the internal property by synthesizing _mutableChildren and using key “children” in -mutableArrayValueForKey:.

How this works is described in Accessor Search Pattern for Ordered Collections.