const data = [
  {
    id: 2,
    children: [
      {
        id: 1,
        children: []
      }
    ]
  },
  {
    id: 3,
    children: [],
  }
]

const getIds = (data) => data.map(d => [d.id, ...getIds(d.children)]).flat()

console.log(getIds(data))