Some multipolygons have thousands of children out of necessity because they are named features. But forests are not named features and as such their geometry should be simplified in such a way that it is easier for editors to maintain it. Also, not all nature reserves are completely covered with forest, if you take a look at Ba Vi national park you can see a road cutting through two big chunk of forests, so mapping the forest as one big multipolygon with the boundaries as children is wrong. It would be better to map the forest along and not glued to the national park.