Traveling in the UK and Europe – visas required

Well, it's going to be confusing.

Basically, my wife, my first son and I are British citizens. We moved to Australia a few years ago (my visa is that I am a permanent resident in Australia, with my wife and my first son).

My second son was born a year ago here in Australia. He is an Australian citizen.

We are going to visit family in the UK in a few months and need to know what visas we will need, if any. By ignoring Brexit, of course (if they manage to pass it before, and who knows what form it will take).

Now that my wife and my first son are still British citizens, I guess we do not need visas. However, I think we will have to ask something for our second son.

In addition, we plan to visit Europe while we are there. Once again, as British citizens (while the UK is still part of the European Union), I imagine that my wife and my first son will be fine, but that we may have need to ask something for our second son.

Am I right to think this (and also that as a family unit, we will have to enter the UK via the queue "coming from outside the EU? "at our destination airport), and if so, what should we ask?