Day trip London to Windsor to Oxford

Oxford might be kind of a dry choice for teenagers unless they are into gothic architecture and museums and there is plenty of that in London. A more interactive choice that they might enjoy is Hampton Court. Take the train out and the Thames boat ride back. Hampton has different tours you can take that feature the different eras of the Palace. I especially enjoyed the Tudor tour. I've heard people advise against driving in Oxford as parking is hard to find. If you do rent a car and want a day out, the Cotswolds are not that far.
Blenheim Palace will give you an idea of aristocratic life style. I've not actually been to Windsor...but if you fly into Heathrow you'll probaly fly over it.

Are the teens into Harry Potter?

Take a look at this site, London Walks. Look at the list on the left side of the page. Look at "Day Trips From London" and "Harry Potter". http://www.walks.com/ One tour is "Oxford and the Cotswolds". You asked, "If we rent a car rather than taking the bus or train would we be able to do Windsor and Oxford in the same day, knowing it would be a long day?" Yes, but you wouldn't have a lot of time in either one. And you do not specify where you would be picking up the car. If you were thinking of driving from London to Windsor, that's a lot of traffic to deal with. You are making a trip difficult that should be easy. Just my personal preference, I would not rent a car for those trips. I would prefer a round trip train to Windsor, and one of the London Walks trips to Oxford. Not necessarily on the same day. There is so much to do in both locations. You will want to take your time and walk around Windsor and Eton after touring the castle. Likewise, you will want plenty of time to look around when you go to Oxford. You don't want to miss one of the most fun things to do in Oxford, which is punting. That takes time. Can't be rushed. Must relax and enjoy it.

Parking in Windsor is very difficult, a few small (expensive) parking lots- when we were there they were really full. A lot easier to take the train. You will probably want to spend a whole morning there, there is quite a lot to see in the castle- particularly the Changing of the guard and St georges Chapel; the town in the immediate area of the castle is very attractive, you can also wander down to the river and go into eton A lot to see in oxford, I dont try driving/parking there anymore. Found the parking quite difficult and traffic wardens quite diligent Oxoford by organized tour will probably get you into more of the sights much more efficiently. However the train ride there is reasonably short

Windsor, as noted, can be very very hard to park in. I took my brother and his daughter there a couple of years ago and could not find parking anywhere on the street or in a reasonable lot. We ended up driving past the Castle and going elsewhere. Oxford is another place that it is very expensive and difficult to park. I have to go into Oxford regularly on business. The council is very anti car and Green. Street parking is a maximum, if you can find ANY, of 2 hours and pay and display. Parking near the colleges in the centre is £1 for half an hour and for an hour it rockets to £2.50, exact change. The parking structures are not cheap, being often as expensive as on the street. You could use Park and Ride but surely that negates the concept of hiring a car? The reason I asked about Pottermania (unanswered at last reading) is that the Colleges frequently would remind Potter readers of Hogwarts, and some was filmed in Oxford. If you are on your own you should realize that most Colleges are available to visit but only in the afternoon and there is a charge, and never around exam time. What is it that caused your kiddos to request Oxford? Brian is right about the traffic wardens. They have been privatised and are KEEN to issue PCNs (tickets). Don't have any part of your car over any line, or too far from the kerb or even a second over time, and if you park where you are not allowed, for example in a reserved or permit space or on a yellow line out of hours or a double yellow line anytime expect a big fine and a clamp.