This place is in the perfect location! So convenient on Gamla Stan and very near everything you want to do. We stayed with our kids ages 3 and 5 and loved it. 10 min walk to a cool playground, very near restaurants, bakery, grocery store, etc. Just a couple minutes from a ferry dock that took us over to the Vasa and Junibacken Museums (recommend both!).
The bedrooms downstairs were great. They have no windows and are very dark--perfect for my kids who sleep best in dark! Great for naps as well! For those with young kids, do note that the stairs curve and don't have any sort of door or gate. We moved a chair in front of them each night so that my 5 year old wouldn't go up them in the dark when she needed us. If you have a crawler or toddler, you'll need to be careful with the top of the stairs as well.
Beds were comfortable and bedrooms and living space had plenty of outlets.
Bathroom is upstairs and tiny. Shower has a shelf for items, but no place to store toiletry items at the sink. I suggest a bag you can hang on a hook with your items. Kitchen was adequate for our needs and had very basic cooking items. Very little counter space, so don't plan on large meal prep.
Keyless entry was a bit tricky to figure out, but did make our unexpected late arrival due to delayed planes easy as we didn't have to coordinate for key pick up. Taxi was able to get right up to the door. On departure we walked to the train station, about 20 minutes (partially over cobblestones).
Gamla Stan is the absolute perfect location to be!