Hi Robin,
Yes, like Andrii said, you can't do that.
But if for some reason, you will only have 1 child record associated with your parent record, or even if at execution time of any plugin/worklow of yours, you know which child record you need to use at you email template, you can create a lookup field
in the parent record that refers to the child record and associate them, with this information in hands you can use any information of the child record at the email template.
Regards,
Henrique Silvestre Souza