We stayed, with a nine month old baby, for two weeks at Helena's apartment. As previous reviewers have mentioned Helena very kindly let us park our pram at the bottom of the stairs.
The apartment is beautifully presented and very stylish, with lots of unique art pieces and quirky touches. There are two bedrooms (one really big main bedroom, and one smaller off the living room), a small living room with a bean bag and sofa and a covered balcony, along with two small bathrooms and a kitchen. The layout is ideal for a baby - with a small office space attached to the main bedroom, separated by a wall and door with a travel cot. This means you have separate space without taking up a second bedroom. However, the apartment is up three flights of stairs, so you have to be ready to carry your baby up a lot of stairs - it's good for you! A baby carrier is essential, both for the stairs and for Lisbon because of the cobbled streets and hills.
The apartment is in a quiet residential area, with good transport links. Helena was very welcoming and available for our questions, because she lives downstairs we often bumped into her coming and going.
A previous reviewer mentioned the noise from the neighbours above the apartment. There was some noise (footsteps) from the family above in the early evening, but it didn't go on too late and they seemed to be away for weekends. It didn't disturb our baby.
It was very hot when we stayed - 34 degrees for three days - and the apartment got very warm (29 degrees according to the baby monitor) so if the heat bothers you a lot, then you might want to think about staying somewhere with air conditioning. However, I think this kind of weather in late May was a bit unusual.
I'd certainly recommend the apartment.