
“Congratulations on your bundle of joy” is fine. It is also what everyone else wrote. New parents are tired and overwhelmed, and the messages they remember are the ones that say something only you could say.
Welcome to the world, little one. Your parents have been the kindest people I know for years, and I cannot wait to see who you grow into.
To the new parents: you are going to be wonderful. I will bring dinner on Thursday. Do not argue.
In ShareGreet you describe your message and the AI writes a short illustrated story. For a newborn, a Storybook or Watercolor style is gentle and warm. The result is a link the new parents can open one-handed at 3 a.m.

Many people add a wish addressed to the baby, which new parents often keep. Good ones are specific: “may you have your mother's curiosity and your father's patience” reads better than “may you have a bright future.”
A story greeting works well here because it does not demand a reply. They can watch it whenever they have a free moment, which for a new parent might be 3 a.m.
If the baby's grandparents speak Kannada, Tamil or Spanish, a narration in that language reaches them as well. See South Indian language greetings and Spanish and Portuguese greetings.
Storybook and Watercolor tend to feel warm and gentle, but you can pick any of the 11 styles.
If the parents share one, yes. You can upload your own photo for any scene.