I want to use 2 imacs (one with FCPX) in same house with Airport Extreme
Your Internet connection has nothing to do with the performance of your hardware with respect to video editing.  The only place it will come into play is when you are transferring data files over your network.

If this is of concern, the answer is simple:  wired is always better.  With wired Ethernet, you're not sharing bandwidth like you would with WiFi.

That said, if you aren't transferring files over the network and all the work will be done locally, it won't matter if you are going WiFi or hard wire.

(IMO) I would hard wire both my iMacs to the network and use the WiFi for convenience like your daughters' MacBooks, iPhones and iPads.  It may cost a bit more during installation, but the reliability and stability will pay dividends in the future.