The Sims 2 Guide and FAQ

1.3.2 Changing Child Sim Personalities
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You can change your Sims personalities with the Encourage option. This can only
be done by a Sim in a more advance stage of their life cycle. This means that 
an adult can encourage a teen or child, and only an elder can encourage all 
Sims.

  Tip - The bigger the difference in the age the more and faster the encourage
        option works. So the best encourage would come from an Elder on a Child.

The good thing about the encourage option is that when a Sim is encouraged in 
one section of their personality, say neatness, and they gain a point in 
neatness, no points are taken off any of your other personality section.

The way that the encourage options works is that the older Sim that is trying to
encourage the younger Sim must have that least 7 points in that personality 
trait to raise the younger Sims rating. If you are trying to lower the rating of
a personality trait the older Sim must have 3 personality points or less in the
trait you are trying to change.

The important thing to remember in trying to use the encourage option is that 
both parties must be in a good mood. The change in your Sim will also happen
faster depending a couple of things. The first is the relationship, the better
the relationship between the two Sim the faster the encourage works.