I would agree with you. Zero is less than 1 on the number line and we use one digit to express the quantity zero. I couldn't find any official reason to back this up though.
What is the smallest single digit number, if the question is for a third grade student? I think it is 0, because In the decimal system 0 is a digit. but my sister's teacher taught her it is 1. in the question it is not mentioned whether it is whole number or natural.