If you decide to visit the town of Sintra in Portugal, then the suburb of Lisbon, then you should be prepared for the fact that it is basically not a resort, but a small historical suburb with castles and dense forests. In the western part of the region, there is an exit to the Atlantic Ocean - Cape Roca is the most stock point of Eurasia. The shore there is quite rocky and not adapted for swimming, although there are still a couple of beaches and there is a tram line to the embankment through the entire city. For the most part, tourists go to Sintra to look at castles and historical sights. If you are not interested in you, you can rest in October-early November, the beach holiday season will end, tourists will become smaller, the air is cooled, and prices will fall a little. In order to see all the beauty of nature, on the journey it is worth going to May-June, when nature has already woke up, but it's not so hot on the street, so that you can calmly walk and see everything, or in October-November, when not very Cold, but not hot. In the spring, the time of the year begins to influx tourists.
Fires around the castle of foam
Savoy near the Mavrov Fortress
These trees are growing in the Sintra dense forest
I do not know whether it is worth going there with young children, because excursions and walks there really tedious. I will say on my own experience, long-term hiking through the Sintra Forest even not every adult sometimes withstands. If you are going exclusively to see the historical sights, then you will probably be as interesting to visit the town's surroundings where you will see the Kapuchin Monastery and Villa Queen de Rigoleiro.