I think many airport transfer companies offer a minibus option for large groups.
I don't understand wanting to Uber or whatever out to Windsor when the train is so easy.
I very much doubt you would fit into two Ubers for that trip, with so many of you. Most would take three of you, four would be a squeeze. Train will take all of you, in comfort.

You’re on the right track (no pun intended ) but a couple of things in your plan can definitely be simplified, especially with a group of 10.

First, from Citadines Trafalgar Square you’re actually closer to Waterloo than Paddington, and for a big group the direct Waterloo - Windsor & Eton Riverside train is usually the least stressful option. No changes, everyone stays together, and it drops you about a 5–10 min walk from the castle.

The faster Paddington route (via Slough) is great on paper (≈35 min), but you’d need to:

• get 10 people on the Tube to Paddington
  
• change trains at Slough
  
• keep everyone together between platforms
  

That’s doable, just more logistics with kids in the mix.

About stations:

• Windsor & Eton Central (Paddington route) → closest to castle entrance
  
• Windsor & Eton Riverside (Waterloo route) → slightly longer uphill walk (~10 min)
  Both are perfectly fine visitors use both every day.
  

Tickets / price:
For London–Windsor this is a short commuter-type route, so advance fares are usually limited prices are often similar whether you buy ahead or on the day (especially off-peak after 9:30).

With 10 people, what I’d personally suggest:

• Just buy Off-Peak Day Return tickets from Waterloo on the day (or at SWR/Trainline online if you prefer)
  
• Ask at the ticket office about GroupSave (UK trains often give one-third off for 3–9 adults; they can sometimes split a group across tickets)
  
• Aim for a train around 9:30–10:00
  

That Omio quote ( $92 for 9) actually sounds broadly in the normal range, but you’re unlikely to save much more by hunting the main decision here is really route convenience vs speed.

I'm convinced we can do the train from Waterloo to Windsor, thank you.
Also, Kt thanks for the taxi, large capacity from the airport.


I just received info from BA that our return flight will depart from Terminal 3. My husband and I have rented a car from Terminal 5 and were planning to stay at the Holiday inn right there. How difficult is it to go from Terminal 5 to 3?

Beware the word Taxi in London

https://www.justairports.com/ is a good source of transport and would cope with your gang
Black Cabs (the shape is the critical thing in London rather than the colour) but is what you hail in the street. Once seen never forgotten
Uber... nah
Some Taxi services are owned by some nasty people, some not, I tend to not use

Note that some of those children will require adult tickets for the train. I'm not sure what the cut-off age is, but certainly the 17 year old and maybe the 15yo.