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.
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.