Heleen's place was great!
We stayed with our two year old, who slept in the smaller bedroom, and our 4 month old, who slept in the big room with us in the crib Heleen provided. During the summer, I recommend bringing an eye mask for the person staying in the smaller bedroom as the daylight lasts a long time and the curtains in that room are not as dark as the larger room's.
Heleen's son welcomed us and helped us figure out good places to eat and even offered babysitting availability should the timing work out. Our kids turned out to be too jet lagged for us to want to foist them on anyone else so we didn't take him up on the offer.
The location of the apartment was great, just one block to tram and bus stops. There were several affordable grocery stores and plenty of tasty restaurants right nearby.
As with all old buildings in Amsterdam, there is no elevator and the stairs up to the apartment are quite steep, but the railings were sturdy and I didn't feel concerned even carrying our baby. There was a place on the landing below to leave our stroller so we didn't need to cart it all the way up.