First, Pixar has yet to make a film about penguins.
Happy Feet was made by Kennedy Miller Productions, same group that was behind Babe.
Surfs Up was by Sony Pictures Animation.
Madagascar (where a team of four penguins were supporting characters) was by DreamWorks Animation, the guys behind the Shrek films.
The closest Pixar has come in making a film including penguins was Wheezy, a single broken penguin squeeze toy in Toy Story 2. The film was not about Wheezy.
Second, Pixar, for the most part, makes films about communities, and how individuals relate to the communities they live in.
This breaks from the Disney tradition, where the animated features tend to be about broken families trying to repair themselves. (Broadly speaking, of course.)
Toy Story was about a community leader reacting (badly) when a charismatic outsider joins the community.
A Bug's Life was about three communities: a farming community terrorized by a criminal gang, rescued by a roving band who help the farmers learn self-reliance. (Think The Magnificent Seven with additional legs and wings.)
Monsters, Inc. was about mid-level leaders of a community discovering and reacting to it's corrupt leadership.
Toy Story 2 - about loyalty, and the re-discovery of self-worth and heritage (it's villian rejecting same).
Finding Nemo - a father searches for his son, and in the process encounters several different communities and discovers how his own place in the world has value.
The Incredibles - about what happens to a family when the community it was a part of has been dispersed and destroyed.
Cars reverses the Toy Story tale, and focuses instead on the charismatic outsider learning to relate to his new-found community.
Ratatouille - again, the charismatic outsider, this time learning how to become a leader in a community of chefs.
Wall-E - hell, it's about an entire species that has lost it's way, and has to learn how to become a community in order to find it's way again.
These films are not identical. They are telling variations on a theme, the theme of the individual's place within a community, but there has been a wide variety of stories to tell within that theme.
Up should be interesting, as it does not tell of a community (unless you count as it's focus a pack of dogs, which from everything I've read is not the story's focus). As such, it makes for a departure for Pixar.
These, by the way, are not children's movies. Their themes go beyond that of the simple "cartoon." They are thus movies that anyone, from any age group, can enjoy, suitable for the entire family, and capable of engaging everyone within the family at their own level.
Too bad so many filmmakers have forgotten how to make such films.