@london101asap I get anxiety so I understand it can feel overwhelming but
it's really easy to get around London!
For the Tube - watch some videos, here's an example so you know what to expect
The tube is pretty quiet at the moment because it is August, everyone is on holiday! You can avoid between 8 - 9.30am and 4.30 - 6pm but you won't need to, it's not very busy at all!
You won't get separated but have a plan. Whoever is left behind stays on that platform and the others come back to get them. You can get a phone signal now on many parts of the network so you can always use your phones if you need to contact each other.
Excel isn't particularly full of the sights, but it is easy to get to and from. London is BIG! Make a plan of what you want to see.
Big Ben, Changing of the Guard, Houses of Parliament, Buckingham Palace, Westminster Abbey? All near Westminster tube station
Shopping? Piccadilly, Oxford St, Regent St, Carnaby St are all close together.
The Museums V&A, Science Museum, Natural History Museum are all walking distance from each other and then you could have a nice walk in Kensington Gardens and visit Notting Hill.
But try not to cram too much, pick a couple of itineraries and follow those so you don't wear yourself out.
My 11 year takes the tube to school every day and he manages just fine :) Do some preparation and you'll all have a fantastic time!