My husband and I stayed at Yumi's place with our 2 year old for 8 nights, and we would highly recommend it! It is 1 minute walk from Yotsuya-Sanchome subway station which is only 3 stops from Shinjuku Station, and trains arrive every few minutes. 7/11 is at the end of the street and the supermarket is only a few minutes walk away. We felt really safe in the area, and the apartment complex itself is quiet.
The apartment is very welcoming and light, and has everything you need to cook and do laundry. Yumi has also provided a big stack of travel brochures and information which came in very handy for us!
We also got to meet Yumi briefly who was very friendly and welcoming.