create a variable out of a child node?

Question

I get an error, set cell on null instance:

extends Node2D

var randNumber : int 

func _ready():
    randomize()
    randNumber = randi() % 100 + 1

    var wall = get_node("../TileMap")
    var grass = get_node("../TileMap2")

    if randNumber > 50:
        wall.set_cell(0,4,-1)
        grass.set_cell(0,4,0)

        wall.update_bitmask_area(Vector2(0,4))
        grass.update_bitmask_area(Vector2(0,4))

I also tried with: - after extends Node2D, same error

onready var wall = get_node("../TileMap")
onready var grass = get_node("../TileMap2")

so... how to put a child node in a variable?
I know, I could just go in and do $TileMap.set cell and $TileMap.updatebitmaskarea and it would work but I'm used to put everything in a variable.

Comments

If TileMap and TileMap2 are childnodes of the node with this script, you should leave out the ../ and use get_node("TileMap") and get_node("TileMap2").