src/Models/SectionModel.php
public function getDirectChildren(array $filter = [])
    {
        return static::query()
            ->filter($filter)
            ->filter(['SECTION_ID' => $this->id])
            ->select('ID')
            ->getList()
            ->transform(function ($section) {
                return (int) $section['ID'];
            })
            ->all();
    }
public function getDirectChildren(array $filter = [])
    {
        return static::query()
            ->filter($filter)
            ->filter(['SECTION_ID' => $this->id])
            ->select('ID')
            ->getList()
            ->transform(function ($section) {
                return (int) $section['ID'];
            })
            ->all();
    }
Get IDs of direct children of the section.
Additional filter can be specified.

@param array $filter

@return array