Do it!
Get the 139 bus from Waterloo towards Golders green, get off by Bond St. Will oy cost you a single fare on your oyster card, and the route takes in a good amount of the lights.
Winter Wonderland is good fun if your kid likes funfairs, but is £££. Is only go off peak. Will be rammed otherwise. Not sure if schools have broken up by then.
Shop windows are looking great - Fortnum and Mason, Harrods, Cartier, Hamleys.
Lots of beautiful trees (e.g. at Leadenhall Market, by Charing Cross tube, Covent Garden one is stunning and huge. Bond Street area looking great too.)
Ice Skating - I'd recommend Somerset House and Natural History Museum – last year for the latter.
Southbank boasts an Xmas market but don't bother. It's just food. Trafalgar Squares one is a short strip if the usual baubles, candles and German sausages, wasn't rammed today (Saturday, peak time).
I think the lights are well worth a visit.