A birthday message for a child needs to fit an age range that changes faster than almost any other category of relationship — what works for a five-year-old reads as babyish to a twelve-year-old, and what works for a teen can go completely over a toddler's head. Here's how to calibrate by actual age, plus who the message is really for at each stage.
A one-year-old obviously won't read or remember the message — this one is entirely for the parents, who likely will keep it. Focus on them as much as the child.
Toddlers won't grasp a written message directly, but they'll respond to enthusiasm read aloud, and parents will appreciate something simple and warm rather than elaborate. Short, cheerful, and focused on fun works best.
At this age kids can read (or have things read to them) and genuinely enjoy being addressed directly — a message that talks to them, not just about them to their parents, tends to land better. Keep vocabulary simple and lean into excitement over sentiment.
Tweens are starting to develop real interests and a sense of humor of their own — a message that references something they actually like (a game, a show, a hobby) lands better than generic enthusiasm at this stage, and shows you actually know them rather than just their age.
This is the age range most likely to be mishandled — treating a teenager like a young child (excessive exclamation points, babyish language) can come across as not taking them seriously. A calmer, slightly more grown-up tone, similar to how you'd write to an adult friend, generally lands better than an over-the-top "kid" message.
When it's your own child, more sentiment is not just acceptable but expected — this is one relationship where warmth rarely oversteps. A message that will mean something to them later (kept in a card, or attached to an Interactive Experience they can revisit) is worth the extra effort a generic line wouldn't need.
Kids' birthday messages often get paired with a gift, and it's worth keeping the message about them rather than the item itself — "hope you love the gift" centers the object, while "hope today is a great one" centers the day and the person, with the gift as a bonus rather than the point.