Drawing two objects next to each other will
make children think about size and proportion.
For differentiation you could have two simple
objects near each other or touching for a more
challenging experience.
Objects Overlapping are more challenging as children are not used to this and they have to really think about how they will draw things that are not there.
The best way to help children is let them
draw the objects first and then talk about
how they can improve the drawing. Then they can
do a second drawing making all the improvements
they talked about. It does take time to achieve this
but patience and practise is pretty much all you need.