What a nice place to stay. I stayed for three nights with my mother and sister in early October 2018. The flat was well equipped and playful in its decor.
A few people mentioned street noise and yes there was a little bit of that as it is right over a pub. The noise was only when people were outside talking, not constant from inside the pub. This may be different in the summer with windows open. I was never woken up by this noise, just noticed it while reading in bed. There was also a little bit of hallway noise but again, I only noticed it when I was awake.
The location for tourists is pretty ideal. There is a jump on/jump off stop very close and you are about a 2 minute walk to the bottom of the Royal Mile. It is a 12-15 minute walk to Waverley Station and about the same uphill to Waterloo Place bus stop area.
There is a tea shop downstairs and a Scottish Pig Roast place across the street that were never open when we were “home”. Hopefully you will keep different hours than us and take advantage of them both. Our Uber driver said good things about the pig roast in particular.
Our hosts were very responsive to any questions that we had. It looks like they are equipped to incorporate children in the flat as well.