Yes, see "Flights from the US" at the bottom of the Travel + Hotel section

Renting a car isn't necessary, but is an option if you want the freedom to drive around the area. Uber/taxi is available in the area and public transportation is great for getting around. You can definitely rent a car from the airport, just note that the default will be a manual car, remember to drive on the left side of the road, and be prepared for lots of roundabouts!

Old Windsor is a smaller town about 3 miles from Windsor.

If you will be traveling with your little ones, we would be happy to see them on Friday and Sunday, but as our reception likely will be later in the evening on Saturday, would prefer to keep it 14+. We know finding childcare overseas may be difficult, and are seeing if we can find any solutions to suggest. Details to come.