With only 11 days, personally I wouldn’t head to the Algarve at that time of year. I have been in March and it’s still cool with some showers. The beaches on the Silver Coast much nearer to Lisbon are just as good and there’s far less travel time. The weather won’t be much different. I like Foz do Arelho and Sao Martinho do Porto. I am not keen on Nazare. Silvercoastwalks.com details walks in the area and you could join their free weekly walks of mixed nationalities but using English as a common language.
If it must be the Algarve, Alvor is one of the most family friendly resorts and Luz. The two coastal places that have some character are Lagos and Tavira.
That’s still very much off season - there will be more snow birds than families there as it’s not school holiday time. The sea is cold even in August, particularly at the western end and this end can be exceedingly windy. The beaches north of Vila do Bispo are the most stunning but you would need a car to get there.
I would avoid Albufeira, Praia da Rocha, Salema, Faro, Vilamoura and Altura as they are either closed for the winter or less family friendly.
