In the interactions between Sophia Lillis’ Beverly and her father, played by Stephen Bogaert, he strokes her hair in an immensely creepy way, and it’s immediately clear to the audience that their relationship is anything but healthy.
In Stephen King’s book, Beverly Marsh keeps her long red hair throughout her childhood, but as Andres Muschietti pointed out when I spoke to him at the IT Los Angeles press day last month, that was something he wanted to change in his adaptation.
Bev cutting her hair in the bathroom is a clear rejection of her father’s advances, and it becomes representative of her deepest fear when It uses presumably those same shorn locks later to trap her in the aforementioned red liquid explosion.