When feet turn inward — a tendency referred to as walking "pigeon-toed" — doctors call it in-toeing. When feet point outward, it's called out-toeing. (from kidshealth.org)
How to express "In-toeing & Out-toeing" in simple terms to a child?
Is it OK to say:
Please do not turn your feet in or out when walking, keep your feet parallel with each other and point them straight forward!