We've traveled with 4 y.o. girl. She slept on the second-floor, in a separate room. She went down on a staircase without any help. But I do not recommend to book this flat if your child is under four.
We loved the location. It is near tram and metro stations. Supermarkets are in the neighborhood, 5 min walking.
Coffeeshops, cafes.. are also quite near.
The closet playground is about 10 minutes walking (but closed in 7 pm in winter time).
The apartment is quite small, but cozy, great window view. You can use dishwasher, washing and drying machines.
We spent 7 days in Prague and love our vacancies!