I used to live in London, but I moved out a bit to buy a house, I went in yesterday and it was fine. Busy, but not impossibly so.
If your Mum isn’t very familiar with London she needs ‘a plan’. Although central London isn’t actually very big, you can walk for miles without seeing much if you don’t know where you need to go or how to get from one place to another.
As others have said, Oxford St, Regent St, Bond St are all very pretty. I think the kids will like Oxford Street lights this year. Selfridges windows were ok, mostly a bit adult & arty, but a couple of cute ones. Personally I loved John Lewis as it’s all about the Monster in their advert AND there’s a Monster House on the 4th floor and the Garden Terrace on the 6th floor with beautiful views of London (and cocktails if you want them! But you can just go up to look around). Liberty is a nice looking building, but not particularly Christmassy & I don’t get Carnaby Street this year - they’ve gone all out with a carnival theme. Very odd & not at all Christmassy.
The Tree in Trafalgar Square is worth taking the kids to see & there are Carol singers there from 4-8 every night this week. SCHEDULE HERE
The Southbank (along the Thames under the London Eye) is good (they need to go into the Rekorderlig Cider Bar’ area, it’s basly advertised as it just looks like a bar but it isn’t.
Covent Garden is usually nice for kids too, but I didn’t make it that far yesterday.
...and of course the view as you walk over Westminster Bridge is lovely.
If the weather is crap the busses are a good way to see the lights. Just tell her to say the 11yo is 10, then it’s free. (It’s free for 11yo’s as well but you need to get a photo Oyster card & it’s a hassle for a day). She will need a contactless debut card though as you can’t pay cash.