You might want to think about staying closer to St. Pancras rather than central London, which will be extremely expensive for summer. The three adults will also be an issue as a lot of hotel have a max occupancy of 2. I think Hyatt is supposed to go the farthest with point transfers?
If you don't care about cost and want to be in central London, look in the Green Park area as you will have an easy ride to and back to St. Pancras on the tube. There are a number of Hilton and Marriott properties in the area. The Park Lane Hilton is lovely and it's close to Green Park Tube Station (which is on both the Piccadilly and Victoria tube line that connects to King's Cross/St. Pancreas) but its $$$$. I think there is a Hyatt in that area also.
But I would actually consider staying close to St. Pancras/King's Cross or even Euston to make dropping bags and picking them up easier (and it will be considerably cheaper) and then just use the tube to go in and out of central London. I tend to do Hilton for one night stays. I stay at the Paddington Hilton on points all the time and I've stayed at the Euston Hilton a couple times to catch early trains.
The tube is so easy to get around so pull up a tube map and you can get an idea of what will be easy to access from King's Cross/St. Pancras tube lines to maximize your time.
https://tfl.gov.uk/maps/track/tube And just cross your fingers they don't have strikes while you are there! London is becoming the new France with transport strikes.