RecursiveRx.swift

struct Parent {
    let id: Int
    let children: [Int]
}

struct Node {
    let id: Int
    let parent: Int
    let children: [Int]
}

func getNodes(id: Int) -> Observable<Node> {
    switch id {
    case 1:
        return Observable.just(Node(id: 1, parent: 100, children: [3,4]))
    
    case 2:
        return Observable.just(Node(id: 2, parent: 100, children: [5,6]))
    case 3:
        return Observable.just(Node(id: 3, parent: 1, children: []))
    case 4:
        return Observable.just(Node(id: 4, parent: 1, children: []))
    case 5:
        return Observable.just(Node(id: 5, parent: 2, children: []))
    case 6:
        return Observable.just(Node(id: 6, parent: 2, children: [7]))
    case 7:
        return Observable.just(Node(id: 7, parent: 6, children: []))
    default:
        assert(true)
    }
    return Observable.just(Node(id: 0, parent: 0, children: []))
}

func getInnerNodes(n: Node) -> Observable<Node> {

    if n.children.count > 0 {
        return Observable.of(
            Observable.just(n),
            n.children.toObservable().flatMap(getNodes).flatMap(getInnerNodes)
        ).merge()
    }
    return Observable.just(n)
}

let parent = Parent(id: 100, children: [1,2])

parent.children.toObservable().flatMap(getNodes).flatMap(getInnerNodes).subscribeNext {
    print($0)
}