We don't necessarily have to agree with their assessment. Some children do and others don't.
Let's take the example of parents who are mostly negative and expect hardships and lack of success. The child might believe them and therefore create an expectation of hardship for themselves, or the child might choose to disagree and create their own expectation. This is how millionaires can be born in slums and how paupers can be born in mansions.
This is also what gives rise to teenage rebellion and what the world in general calls "attitude problems". This is how children who strongly refuse to accept the normal limitations of the beliefs people hold around them are labeled. There is really nothing wrong with these children except that they want their freedom; they want the freedom to believe and do as they wish.
So the expectation creates a focused thought that calls to a result in keeping with it's vibration - like attracts like. Therefore, the results are invariably in keeping with the expectations, and a belief is born. The belief reaffirms the expectation and makes it stronger each time.
The old adage "learn from observation" is obsolete, we need to learn from our own experience based on positive expectations.
Sometimes, people think that they are setting up positive expectations, but deep down they have come across some information or someone else's opinion that they considered to be true or carry a lot of weight - now their seemingly positive expectation is tainted. Again their experience matches the expectation, and their belief system is formed.
When you know what you're doing and why you are getting the results you are getting you can alter your expectations. For some people this is a slow, step by step process whereas others can flip a switch and it's done. Either way, the bottom line is that it can be done - and that my friends is the only important thing.