If you have accidentally deleted your child from your account you will need to add him/her back via your profile. Simply go to your profile (top right of the screen) and click the 'View Profile' button. Select 'Manage Pupils' from the list on the left hand side of the screen, then click the 'Add Pupil' button and fill out your details. If you're already logged in why not do it now: go to your profile?
For further information on managing your profile generally, click here.