In the Netherlands it is a criminal act to keep a child from the person(s) who has custody over the child. It is called ‘withdrawal from authority’. This criminal act is described in article 279 of the Dutch Criminal Law.
The Dutch prosecutor’s office has written a guideline about what kind of punishment it will demand in criminal cases about withdrawal from authority. You can read this guideline on the website of the government. The fourth category of cases refers to international child abduction.
In 2019 the Dutch Appeal Court in The Hague has imposed a sentence to a father for international child abduction. He was sentenced to imprisonment for a period of 42 months. He had taken the children from the mother when they were 3 and 8 years old. After that he had taken them to other locations and had changed their names. As a result the mother did not know where the children were for a period of 11 years. You can read this decision on the website of the Dutch courts.