The answer to this depends on exactly what you are doing.
Generally the nodes are drawn in order, that means the first child is drawn first, then the second one etc.
For isometric games there is also the Y ordering where children of the tilemap are drawn depending on their Y-value on the screen. Objects that are closer to the bottom of the screen are considered in front of objects further up.
Then there is a Z-value for all objects that inherit Node2D that you can use to move some objects to another layer so to speak.
Depending on what exactly you want to achieve you should choose one of these three methods.
Edit: dc1a0 reminded me of CanvasLayers. They have a property that is just called "layer" which kind of works like a Z-value. But additionally they can be made independent of the camera (good for a HUD) or to move faster than other layers (ParallaxBackground). More infor here.