Personalized, on most product pages, means a name in a sans-serif on a lunchbox. Children notice the name once. What they reread is a scene where the habit is theirs.
Under five
Keep the world inside one room and one adult they know. A birthday book at this age is the party they are actually having, not a costume theme. The birthday story asks for the age so the chapter does not accidentally write them older.
Five to eight
They can hold a short trip if it comes home. A bedtime adventure should leave by a door you name, solve one trouble the child solves, and end in the same bed. More plot is not a gift at 8 p.m.
Nine to twelve
They will reject a baby version of themselves. Use the age they are turning and a habit that is current, not one from the toddler years you miss. Siblings belong in the scene only if the child would notice their absence. Say so in the notes. Do not add a moral about growing up. They can hear that anywhere.
Across these ages, print the pages. A PDF on a lock screen is not the same object as a stack of paper on a nightstand.