"When We Grow Up" is a "Playtime Board Book" in the The Land Before Time series. It was illustrated by Robert Sanford and Christopher Fowler, while the cover art was provided by Universal Studios Liscening Inc. The back of the book is blue, and shows the original five Prehistoric Pals plus Chomper. It was published two different times under different companies.
Presumably, it takes place during the events of The Land Before Time II: The Great Valley Adventure, as Chomper is in the book, but is a baby.
The book starts out with Littlefoot, Cera, and Ducky, complaining about grownups, saying that rules are no fun. Spike, Petrie, and Chomper are with them too, and are disappointed. It is revealed that the gang were going to spend the night out in the forest, and have their biggest adventure ever, until the grownups said "Wait till you're bigger." Cera then stands up and says "I am a three horn, and a three horn makes her own rules...I'm going!" The others try to stop her, telling her that she'll get in trouble, but Cera calls them babies, and leaves. None of the gang are happy about being called babies, and follow her, which was Cera's plan all along. When the gang start to catch up with Cera, they hear a gurgling noise. Ducky gasps, saying she knows the sound. At that moment, the clouds roll back and the moon shines down to reveal the sinking sand right in front of them. The gang all see the sand, except Cera, who is charging right towards it. The gang call out to her to stop, but too late, and Cera slips in.
Cera calls for help once she realises that she can't move her two front feet. Spike is the first to think of a plan; he pushes a dead branch towards Cera, hoping she'll grab on. Cera reaches for it, but her two unstuck feet slip, and she falls back again, sinking deeper than before. A huge shadow suddenly appears and Cera is swooped up in its jaws. The other children close their eyes and tremble in fear of the shadow, but when Cera is set down gently, Littlefoot hears his Grandpa's familiar voice. Grandpa Longneck tells the children that until they learn to follow simple rules, they will never see or understand all the secrets of the Great Valley. All six children realize they have grown up a little this night, but are glad they aren't too grown up for a nice ride home.
- Baby Chomper is in this book, despite the story not happening in the second movie.
- Chomper is with the gang when Grandpa Longneck saves Cera from the Sinking Sand, but he doesn't comment about a Sharptooth being with them.
- Like the other Land Before Time "Playtime Board Books", This book has two slightly different versions. One where the Land Before Time logo on the cover is colored, and one where the logo is silhouetted in blue (the one in the main picture). Both versions have different ISBN numbers, and were published by different companies at different times; the colored-logo version being newer.
- Strangely, the colored-logo edition of this book has the same ISBN number as the other colored-logo "Playtime Board Books", while the silhouetted-logo version has a unique ISBN number. The cause of this is unknown, as books are intended to have unique ISBNs.
Many of the characters in this book have slight coloring differences compared to their colors in the film.
- Littlefoot's eyes are colored a dark brown in this book, whereas in the films his eyes are more of a reddish-brown.
- Petrie is colored in his alternate colors.
- Spike's eyes are colored red on the cover of this book; in the book itself they are more magenta colored, but not their normal violet color. He is also a lighter green, and his belly is cream colored instead of brownish-orange.
- Grandpa Longneck is designed different in this book. He is a lighter gray, and is less blue-gray than in the movies. He also doesn't have any darker colored bags around his eyes, his back stripe goes to the tip of his nose instead of stopping between his eyes, and his middle color goes over his jaw instead of just his snout.