We stayed at Frederique's in late February 2018 (my wife, my mother-in-law, two sons (1.5yrs and 3.5 yrs) and myself. The apartment was great. We loved the location. There was easy metro access, a great coffee shop around the corner, and tons (literally tons) of fruit sellers and supermarkets (including some organic/natural ones) in the neighborhood. We also like the fact that it was in a quieter, more residential area of Barcelona...this gave it less of a "in the middle of the tourists" feel. That said it is an easy ten minute walk to Sagrada Familia and a five minute walk to the metro so you are minutes from anything.
Also, coming from West Africa, our flight times are not ideal and we had to arrive at 4am on a Sunday morning. Frederique was greatly accommodating and had someone waiting for us at the apartment to hand over keys and go through everything about the apartment (washer usage, heating, stove, etc.)
In closing I will also say that the massive toy closet was a hit with our boys as well :-)
The next time we are in Barcelona, we would definitely stay at Frederique's again.