I think that the Parenting skill (especially as it increases) is in place to ease any disagreements that toddlers, children or teens may have. Also, in the play through by the Gurus, one family was more understanding and nurturing. I think that a parent Sim's personality, aspirations and traits will predict how good a parent they want to be --- and they will become even better parents, as they master the new Parenting skill.
For example, in my main family, the household consists of the 2 children's grandparents, parents and aunt and uncle. The adults are all soulmates, genius, good and ambitious. The 2 children are whiz kid/genius. The children hardly misbehave, and the adults very, very rarely argue. Also, the 3 adult couples are best friends and so are the 2 children. I really believe that already being best friends will help to alleviate any familial problems that may arise. I also have aging off for all Sims, so if you do, and your Sims get along well now, then, I don't think this pack will affect them negatively.
I think that if your Sim family has good aspirations and traits (the ones I mentioned above or other positive aspirations and traits) and they get along most of the time, that this pack will only add a few rare times that misbehavior is a problem. I think that it will add much great gameplay/activities for families to enjoy.