If the other parent with custody refuses to sign the enrollment form, you can ask the court for substitute consent.
In the Netherlands, this is done by means of Article 1:253a of the Dutch Civil Code, which article can be used for all kinds of disputes between custody parents about the children. The request must be dealt with by the court within six weeks. If you can’t wait that long, you can start summary proceedings.
The court will make such a decision as it deems desirable in the best interests of the child. If the child is 12 years or older, the court will give the child the possibility to give it’s opinion. This can be done by writing a letter or by talking to the court. If it is a complicated matter, the court can ask for advice from a guardian ad litem, or the Dutch child protection services.