This is a great proposition for the warmer days approaching.
The praia das maçãs train has one of the most bucolic routes we can find in all of Lisbon, it departures from Sintra and quietly follows its verdant path, until the final (or initial) station of Praia das Maçãs.
To get to Sintra, I recommend to, if you have one, go by car, otherwise take the Sintra line train in Entrecampos, for example, and arrive right in the center of the village. The departure and arrival stop of this singular train is located on the right side of the Olga Cadaval Cultural Center, which is very easy to find.
This is a trip of about an hour, comfortable and not at all risky, highly recommended, and very kid friendly.
Enjoy a late afternoon (yes, I recommend you to do it in the afternoon... Sintra, in that aspect is not like Lisbon at all, in the morning it is cold and very humid!). If you prefer to spend the day at the beach then leave early in the morning, return at the end of the afternoon and also enjoy your lunch in one of the restaurants of sea food and fresh fish near the beach.