Thank you for your suggestions of using Priceline (found some hotels there
too, now I'm comparing the prices), I'll check out LondonNights as well.

I'm currently checking out Venere.com, recommended by Viamichelin.

I have a problem in finding out how much the end price would be, regarding
the child policy:
- some hotels have "children" legend, which I take it they allow children
- I wrote to Venere.com customer service about it, and their response was
the following: "All rooms booked through our website are guaranteed to
accommodate
two adults. Although we do not offer special discounts for children, a
child may stay in the room if at least one person in your party is 21
years of age or older. If you are traveling with more than two guests,
we suggest you request more than one room"

I'm not sure if that means that:
- a child is allowed in the double room with adults over 21 (which of course
we are) free of charge, and in that case, will she get an additional bed (no
cot, she's 3yo)
- a child is allowed in the double room... with a supplemental fee and her
own bed.

I'm now thinking my best bet would be to book a triple room and be done with
it, but triple rooms are hard to find during Christmas period in central
London (I'm thinking Bayswater).

Anyway, I wrote them back asking them the same question, but I'd like to
know your input on the subject, if you have any to offer, it would be
appreciated.