There is no direct concept of a “parent” or “child” in FX - the closest analogues are “args” (i.e. what nodes your current node is using) and “users” (i.e. what nodes are using your current node). Furthermore, “users” is a property derived from “args”, so the only thing you should be modifying is what the args to your function are.
That being said, it’s not totally obvious to me what the graph rewrite you want to do is (where does conv3 go?), but I’d imagine it looks a bit like this (assuming you want to replace conv3 with concat?):

create conv2_1 and conv2_2 from conv2 (probably using nn.Module.add_module)
create 2 call_module nodes that take in conv1 as an arg using fx.Graph.call_module (inserting in front of conv3)
create a concat node with conv2_1 and conv2_2 as input
Replace all uses of conv3 with concat using fx.Node.replace_all_uses_with


It might also be easier for you to use the Subgraph Rewriter API: torch.fx — PyTorch 1.8.1 documentation 2