This is expected. Although the actors can’t be different on different machines they might not be loaded or the order of loading might not be the same so sending a reference to a level object is a no-go.
Try to work your way around the problem. Instead of sending a reference send an the index of the child object or attach an identification and send that “id” of the child object.
Note that in a network environment Multicast event might not work since any of the level objects might not be loaded for any particular client - the client might be in a different place on the map or the client might have joined late (after the multicast)
Use a replicated value Instead that is set on the server. This way, any client can check the value and infer what the action was (which button was pressed) even if they were not present at the time of the event.