How do children draw? Schematically. Badly. And, as it turns out, a lot like how they did 800 years ago.
Anthemius of Novgorod, or "Onfim" in his native Russian, lived in the 13th century in Novgorod. He's one of many authors and artists behind the thousands of pieces of scribed medieval birchbark that have been recovered from Novgorod, and he's unique among them for one major reason: he was approximately seven when he created the eleven pieces of his work we know today. In them, he practices writing by transcribing passages from the Book of Psalms and the alphabet, and he doodles action scenes, usually depicting himself as a knight - often on the same piece of birchbark.
What's striking about Anthemius' doodles is how ordinary and unremarkable they seem - there's nothing about them that couldn't be written today, except for the archaic language. In one, he gives up on an alphabet exercise and switches to drawing a warrior with a spear. He labels the warrior Onfim, making it clear it's meant to be a grown-up version of himself. In another, he draws what appears to be his parents. When drawing people, Anthemius often depicts a pair of legs that flow into being the sides of a body with a defined top and no defined bottom, an artistic style I've seen produced in modern children's art.
There's even almost a speech-bubble convention in Anthemius's art. In one piece, apparently a note passed to a friend, he draws a wild beast, labelled Ya zver' "I am a beast" and apparently intended to represent himself. The beast is carrying a sign which reads, Poklon ot Onfima ko Danile "Greetings from Onfim to Danilo." The sign looks like something that could easily be left floating beside beast-Anthemius and resemble a modern child's attempt at showing a speech bubble. More relevantly, though, we see a kid who is already learning to write at age six or seven and is already proficient enough at it that when he gets frustrated by his lessons and writes a note to his friend, he writes it out properly (if briefly!) and expects his friend to be able to understand it by sight.