Don't know much about the GU and how far you could get depends on how hard you want to work. I did Great Hayward to Banbury in five days on my own but I would not call it a holiday. The North Oxford is straightforward, the south meanders a bit and has lots of swing / lift bridges not so many locks.
The non tidal Thames is easy, all the locks are manned during the day, it's wide compared to a canal and when we did it at Easter it was very quiet. Finding moorings can be a pain, they are not clearly signed in many cases and you often have to pay. We found boatyards full of cabin cruisers, not much space for Narrowboats, so getting rid of rubbish etc. was sometimes a problem.
You should have an anchor and life-jackets are recommended especially for kids. Tidal section you need a VHF radio, mooring and navigation lights and I would suggest some experience. EA have a useful booklet on their Thames website which you can download.
Enjoy the trip, hope you get good weather.
Ken