If you leave Belgium, the Belgian inheritance rules will not apply anymore except for the real property you have here. You will be subject to the inheritance rules of the country to which you move. Whether a Belgian (or a foreign) Will is to be considered valid in another country depends both on the legislation of the country where you live at the time of your death and on the legislation of the country where you have your assets.

That being said, real property in Belgium will always be governed by the Belgian inheritance rules.

If you move to a country that does not have forced heirship rules, you will be able to disinherit your children and leave everything to each other, but you children would be entitled to claim their share on your apartment in Brussels.