Wireless Guest access on routers - how safe is it? Can they be hacked?

Good thinking. DMZ? What that?

DMZ is shorthand for DeMilitarized Zone.  It refers to an area located between two routers.  In this case the DMZ and the guests are in the first / outer router (the one which faces the internet), and your own LAN is in the second / inner router (the one which faces the first / outer router.)

Ok so www ---> guest router -----> wan port of internal router

Correct.  Outward facing router (any wired port) provides a wired connection to inner router's incoming WAN port.

Very well explained

So the "guest" router wireless would be configured as normal and no "guest" wireless feature required.

Like some routers have a <x> enable guest access. But in this case I don't need it - correct ?

That's correct.  Set up one WiFi network on the outer router for guests.  Set up another on the inner router for yourself.  If either router has guest capability, disable it.

So use diff router brands.
What are the chances of someone with vast networking experience hacking into the LAN for the guest router. Not govt / CIA or anything!

Given enough time and effort, I believe anything can be cracked.  The probability in this case is small if the router firmware is kept up to date.