You need to have some dates and more facts, before you are able to search in Belgium.
Did the marriage took place in NY/NJ or in Belgium? are the children born in NY/NJ or in Belgium?
Did you find them in the census lists in NY and NJ, especially in the year 1900?
Because the 1900 census lists not only the year of immigration but also the year of the marriage.
Did you find their ship manifests or if they were naturalised, their naturalisation papers or passport applications?