Firstly, don't spoil your well earned break by fretting over what this parent might say. As a HoD I hear from far too many parents trying to tell us how to do our job. You had a clear objective for setting the homework. If their daughter managed it swiftly and correctly then they can be confident that she has mastered a basic skill which will provide a solid foundation on which to build further skills. If she struggled/got some wrong then it has highlighted a basic skill which she needs to practice. I'm glad your HoD was supportive and I'm sure they will be again if needs be.