When traveling from the United States to the Netherlands (a European Union member state), the customs regulations of the EU apply.
Even if you are slightly under the limit, declaring voluntarily can prevent delays or questions if officers detect large sums during screening. Failure to declare can result in seizure of the funds and significant fines.
Note: U.S. customs also requires declaration of amounts over $10,000 when leaving the country (FinCEN Form 105).