With NetworkX' DiGraph, how to find single input leaf-nodes starting from a certain node?

Taking the following graph:

G = nx.DiGraph()

G.add_edge(1,2)
G.add_edge(3,2)
G.add_edge(1,4)
G.add_edge(2,5)

which visualized gives:

I'd like to find the leaf-nodes which only have incoming-edges from within the "sub-graph" starting from 1.

In my example it has to find 4 but not 5. 5 is a child of 2 which has 3 as second input.

I think it should be something with successors and the in_degree but I'm such a newbie to NetworkX that it is quite hard to find the right algorithm.

Another example:

G = nx.DiGraph()

G.add_edge(1,2)
G.add_edge(3,2)
G.add_edge(1,4)
G.add_edge(2,5)
G.add_edge(4,6)
G.add_edge(1,7)
G.add_edge(7,6)

G.add_edge(1,8)
G.add_edge(8,7)
G.add_edge(8,6)
G.add_edge(4,9)
G.add_edge(4,10)
G.add_edge(5,10)

Here it should find 9 and 6. But not 10 (because of 3 being parent of 2 being a parent 5.

Let's try this bit of logic:

def find_leafnodes(G):
    leafnode = []

    for i in G.nodes:
        head =  []
        if nx.descendants(G, i) == set(): #find all leaf nodes
            for a in nx.ancestors(G, i):  #get all ancestors for leaf node
                if nx.ancestors(G, a) == set():  #Determine if ancestor is a head node
                    head.append(a)
        if len(head) == 1: #if this leaf had only one head then append to leafnode
            leafnode.append(i)
    return leafnode   

Input:

G = nx.DiGraph()

G.add_edge(1,2)
G.add_edge(3,2)
G.add_edge(1,4)
G.add_edge(2,5)

find_leafnodes(G)
# [4]

G = nx.DiGraph()

G.add_edge(1,2)
G.add_edge(3,2)
G.add_edge(1,4)
G.add_edge(2,5)
G.add_edge(4,6)
G.add_edge(1,7)
G.add_edge(7,6)

G.add_edge(1,8)
G.add_edge(8,7)
G.add_edge(8,6)
G.add_edge(4,9)
G.add_edge(4,10)
G.add_edge(5,10)

find_leafnodes(G)
# [6, 9]