In that case, about 3 weeks before you go, go onto the Sky Garden website. It's the walkie talkie building opposite the Shard. You can go up to the viewing platform for free and there's a cafe and a botanical garden inside it. Fantastic views.
We went with our 6 and 3 year old last month for 2 days. Absolutely loved it. Did loads of walking but paid for the Uber boat from Tower of London to Parliament and back again later in the day as our only real transport apart from the day 1 tube to the museums. Was a lovely leisurely ride and...