{
  "foo" => {
    "id" => 123,
    "children": => {
      "0" => {
        "a" => "a"
      },
      "1" => {
        "b" => "b"
      }
    }
  }
}

params.require(:foo).permit(:id, children: { "0" => [:a], "1" => [:b] })

{ "id" => 123, "children" => { "0" => {}, "1" => {} } }