This is a very nice apartment for a family with children from around three or four years old.
The apartment itself good-sized and well-equipped with two large, airy bedrooms on a separate floor. The bathroom is small but everything works as it should, and there are no hot water or water pressure issues. The kitchen includes a coffee machine with a reusable filter, among others, and coffee for the machine is provided. There is a dishwasher, so you don't have to hand-wash everything. The complimentary bottle of wine in the fridge was a nice touch :-)
The stairs can be tricky for the youngest kids, so be careful here and maybe help them up and down, but I wouldn't advise to avoid this apartment for that. Again, just be careful and everything will be fine.
The TV comes with lots of different channels available, including several children's channels, which was very much appreciated. There is also air conditioning, which provides relief for hot days and nights.
There are several small food shops and bakeries nearby, plus a sort of grocery store called Pingo at Apalonia (sp?) train station, where you can otherwise stock up on what you need for breakfast/meals you want to prepare in the apartment. It's not big but you can get all the basics.
The location is also right in the middle of a ton of restaurants where you can hear fado music, although just being in the area gets you all the fado music you need. You can hear it from the apartment. But don't worry; it does not last all night and it's otherwise quiet at night.
The Metro station isn't super close but walkable without hassle, even for a four-year-old. If the steep uphill walk up to the castle is too much for the small kids (or even you), the bus stop to the castle is also within walking distance (closer than to the Metro if I remember correctly).