Parking in Windsor (and Eton) is awful. Hard to find and expensive.
Windsor Castle isn't cheap, and you may as well get your money's worth. It is an easy half to all-dayer. Changing of the Guard, St George's Chapel, Queen Mary's Dollhouse, the walk through the castle in the State Rooms, the Semi-State Rooms, the Royal Apartments, and stopping to look at the world quality art on the walls, the Grand Reception Room, these all take time. These are all indoors, and there is a substantial amount of the castle outdoors. And most people don't even visit Windsor Great Park or anything outside the walls. When I was a member there we returned something like 8 or 9 days and didn't ever see it all.
Then a very long drive down to either Stonehenge or Bath? A long way to go.
The reason the coach trips can push it all together is they only give you glimpses. You can only squeeze so much in and of course on the coach tours most of your time is in the coach seat.
I don't know you, Lynne, and don't know the situation of your family. Many people are driven by cost, and if that is the case then you can save large piles of cash by getting your tickets well in advance - although for May, especially the end of May when the second Bank Holiday occurs, many of the best priced tickets will be gone because the tickets have been on sale for a while. Some other people are motivated by convenience and have the wherewithal to pay for that convenience. If that is you, then most Eurostar trains have some availability up to close to departure. Then again with the Late May Bank Holiday hoving into view availability will be tighter both before and after. But if money is no object there may be more room in Business Premier because the execs and politicos won't be travelling to and from work so much.
You have to check in ahead of departure - around an hour ahead although I advise more around a Bank Holiday - because you go through security then UK departure Border control, then French immigration Border control, and then over to the waiting room before being called to go up the travelator to the platform to board the train. Just 10 minutes (unless it has gone up recently) if you paid top whack.