I found a site which clearly explains children's stages of drawing development:
Ask your child to draw family portraits regularly to keep a record of their artistic development. Always name and date them, and encourage your child to tell a story along with the drawing.
Here are Finn's family portraits aged 2 years 8 months (in a wild mood).
Evie (baby sister).
This is a drawing of Evie with other baby Evie's inside her tummy. She is wearing a nappy.