Criança vs. filho
While both criança and filho can be translated as child in English, they are not interchangeable in Portuguese. The key difference is that criança refers to a child by age, while filho refers to a child by family relationship.
Criança
A1The word criança refers to a child in terms of age, describing a young human being from infancy to early adolescence. It is a grammatically feminine noun (a criança), but it is used for both boys and girls.
Há muitas crianças a brincar no jardim.
(There are many children playing in the garden.)
Quando eu era criança, eu morava no campo.
(When I was a child, I lived in the countryside.)
Aquele menino é uma criança muito educada.
(That boy is a very polite child.)
É importante garantir uma boa nutrição para cada criança.
(It is important to ensure good nutrition for every child.)
O filme é apropriado para crianças de todas as idades.
(The movie is appropriate for children of all ages.)
Filho
A1The words filho (son) and filha (daughter) refer to a child in terms of kinship, meaning someone's direct descendant or offspring. This term is not dependent on age; an adult will always be their parents' filho or filha. The plural filhos can mean sons or children (of mixed gender).
O meu filho já tem 40 anos e é engenheiro.
(My son is already 40 years old and is an engineer.)
Ana e João têm dois filhos: um menino e uma menina.
(Ana and João have two children: a boy and a girl.)
A minha filha mais velha vai casar no próximo ano.
(My oldest daughter is getting married next year.)
Ele é o filho único dos meus vizinhos.
(He is my neighbors' only child.)
Vou passar o fim de semana com os meus filhos e netos.
(I'm going to spend the weekend with my children and grandchildren.)
Summary
In summary, use criança when talking about a young person in general, based on their age (a kid). Use filho or filha when talking about someone's offspring, based on their family relationship, regardless of age. Therefore, a 50-year-old man is his mother's filho, but he is certainly not a criança.