We enjoyed our stay at this apartment. It was clean and well equipped. It's a rental flat rather than someone's home but has all the basics. Good Wifi.
We have 2 children (5 and 8) and although it was fine for them - any younger I think may not be advisable due to the mezzanine 3rd bedroom (which they insisted on sleeping in). The staircase is not small child friendly.
It is on a busy road but that is fine with lots of restaurants and cafe's near by. Also a lovely food market within a 5 minute walk. Sagrada de Familia is a 10 min walk and we got the hop on hop off bus from there as well. Also very close to other Gaudi buildings. Felt very safe to walk around.

The walk into the centre of Barcelona is about 20-25 mins - which our children walked quite happily.

The only thing to note (especially with children or bigger parties) is that there very clear signs about making sure there was no noise between 10pm and 8am which we were slightly nervous about.
Overall we had a good stay!