This is the email address of the police station in Sint Niklaas. Put in the subject line something that will catch their attention.
[email protected]
Write your email in the clearest, most unemotional way you can. State exactly what has happened and give dates, times, names and addresses.
Give them all the information you have, including children's birth certificates and your ID, so that you can prove who you are and that you have a reason to be concerned.
Give all and any information you might have about where he could be i.e. his mother's name and the town she lives in. State she moved recently and if you have it, give her old address.
If you have a phone number for any of them give those as well. Even if they no longer work. Give any family contact details you can. Email, phone, address. That you are blocked doesn't mean they won't answer when the police call.
State the last time you had contact. Explain that you would ask them to do a welfare check to verify that he is alive and ask them to verify that he has abandoned his family. Ask them for his address so that you can claim for child maintenance payments as you have discovered that he has resigned from his job in the UK. Explain that he has left you destitute with 3 children under 5.
Write all this in English. Write it clearly using absolutely clear language. Take your time. Then put it into DeepL for a translation. Explain that the translation is using software and so might not be completely accurate.
The police will do things for you as family, that they will not do for any stranger. When you have more information, others, including me, will possibly be able to help you further. But this first step needs to be yours.