If you are planning to visit a lot of attractions, you could save money by getting a London Pass. Especially if you are staying longer, a 6 day pass is just over £100 for an adult and £72 for a child. You can also get 3 and 1 day passes - a 1 day adult pass is £47, so obviously the 6 day one is better value, but of course it depends how long you are going to be there. In fact even a 1 day pass can be a good bargain if you are planning to visit lots of places in that day.
The pass covers over 60 attractions, including all the famous ones like the Tower of London, Thames river cruise and so on. A big plus is that you can skip the ticket queues at any attraction covered by the pass. You can also opt for one with travel, which covers all your public transport for the relevant period.
A pass is valid from the day it's purchased rather than a fixed number of hours, so make sure yours starts from the morning rather than the afternoon.