Since you are teaching, just give them the measurements and the amount needed. It is their choice to use this information wisely or not. If she is misinformed by others you cannot help her with that. It is the child who will be embarrassed if the parent fails (sigh). You have done your best and further explanations will reap no positive results. Good luck and know that there is always one adult like this in every group (smile). Kudos to you for teaching beginner quilters.