you could send a letter/email/phone call, stating that you would like to see them so you can give them an update and other important daycare information.

if it was just to let them know about days you are going to be closed then a letter address that would be fine. With telling them about you leaving and behavioral issues then you should talk to them in person.

If you tell a family member that you need to speak to the parents they might start asking questions (what you need to tell the parents is non of their business), or forget to tell the parents. Its also not as professional IMO.