We LOVED this place! We stayed for two weeks and already want to come back.
When we first arrived, I was very overwhelmed by Madrid and wondered if we made a mistake deciding to stay in a big city for so long with such a young child (my son is 1.5 years old). However, within a short period of time, I fell in love with Sonia's place. Despite being near a busy location (with LOTS to do!), Sonia's place is very quiet and cozy. The little street it's tucked up in is very quaint, with a little playground and delicious restaurant (Mur--can't recommend their "veggie version" breakfast enough!). We saw the same people/dogs/children day after day and it felt like a sweet little neighborhood, despite being in a big city.
In terms of location, it can't be any better. It's very close to a metro station and nearly every place in the city is accessible. My recommendations are walks in Templo de Debod at sunset (walkable). We did that several times. Upscale tapas and tinto de veranos at Origen (everything was delicious!). And if you have children, visit Piscina Lago (right off the metro) for a swim when it's hot out.
Can't recommend high enough! Sonia is amazing host and very communicative/helpful!