We are a University and when prospective students signs up for an Open Day they can enter their parents contact information. When you click on the register on this link at the bottom you’ll see this message We want to use these parents email address for google display advertising to send them to our website. How can we do this? As they won’t necessarily be aware that their child has entered their email address.
First of all, consider that the data process of minors is lawful under GDPR when the child is at least 16 years old. Under the age of 16 years old, you will require their parent consent (in such case, you can have access to their parents’ email).According GDPR, you can require parents' email but you cannot use it for marketing activity without their consent.
In fact, you need to consider that for targeted marketing activity the GDPR requires the consent of the targeted person so that children and parents need to flag the box wishing to receive your advertising.
In such form, to be GDPR compliant, you should ask for consent from prospective students and/or parents, also adding a box to flag for marketing purposes. You should inform them about the data process in your privacy notice.