If your children are minors, living here in the Netherlands and both of you die, then a legal guardian will be appointed by the Dutch court according to either a wish by the parent(s) stated in a will/deed or filed by them with the Gezagsregister (Dutch custody register).
This law will apply from the day you move to the Netherlands and become a resident. If you are not married, live together and have children, a will and a co-habitation agreement are recommended.
You can appoint a guardian in a deed drawn up by a civil law notary or in your will or by registering your choice of guardian at the Dutch custody register. The guardian who could be a family member or someone else is the person who has authority regarding minor children and he/she raises them and represents them. He/she also administers the children’s inheritance, but in case you think that this person doesn’t have the required expertise, you can appoint an administrator. You can also appoint an executor who will take care of winding up the estate on behalf of the heirs (e.g. taking care of the funeral, paying taxes, selling the house, etc.).