I want to add 10 fields to user object. Shall I add it to User object directly or shall I first create child object of the user object and then add those fields to it? Which one is best approach? any thought?
If the fields that you need to add have a direct 1:1 relationship with any given user, I would say add them to the User object directly and possibly create a new Section on the page layout to house them.
The other thing to consider is that you can't have the User as the Master in a Master/Detail relationship you can only specify a lookup. If you create a lookup from your 'child' object to User, I don't think you'll then be able to see those records as a related list on your User record as you might expect on other objects.
You could however create a lookup from User to a new object which would then allow you to access the object from the link on the User object or in apex via the Relationshipname__r.