What I used to do in such a case is send a WM_COMMAND to the parent, but I had to have known the button id to accomplish this. It's no big deal to find it but it's not generic enough. the WM_MOUSE messages are good too, since they are relative (as I remember?) to the upper left of the parent window and do not steal mouse cursor. If you resize your window however, you are thourougly screwed. WM_CLICK seems perfect, as you can *easily* get the handle of the button, you just have to ask windows nicely for it
I guess it pays to read the msdn library. Thanks Satyric0n!