Samsung Galaxy S8 has updated children mode that is kids Mode in their system and if you will follow old Samsung phone steps then you will face problem in finding that mode.
You need to install it as this is not available on your phone to install Open the app menu and then the folder “Samsung”.
Go to “Galaxy Apps”. > Samsung App Store > find the children’s mode > Touch the “Exclusive” tab at the top of the site For Galaxy Essentials, select All apps Scroll down the list until you see “Kid´s Mode” –
Install this on the Samsung Galaxy S8.
Now You can then set up the child mode on your device. In case of any problem comment on this post.