You can open a minor demat offline by visiting the nearest Kotak Securities branch. Do carry the required document mentioned here to open your account instantly. Click here to find your nearest branch.
Please note that the minor can open a Demat account in India, but with certain restrictions:
In summary, a minor can have a Demat account, but it is primarily used for holding securities, not for active trading.
You may also read this article on Open Demat Account for a Minor