What are the disadvantages of when parent choose career for there children themselves
​

There are many. A parent may not know what their child is motivated to do (motivation is a big factor of work). Another concern is how well they will perform in general with this occupation. As bias as it seems many parents are also selfish with choosing life altering decisions for their children, and may go with something they may enjoy more than their child. Payment can also affect the child based on their job, if the child is also working to support another individual the parents may miscalculate their income which could make it hard to support another person.