Formal:
* Nau mai ki te ao, e taku mokopuna. (This is the most literal translation, but it feels a bit more formal.)
Informal & Affectionate:
* Tena koe, e taku mokopuna. Nau mai ki te ao! (This is a warmer greeting, literally meaning "Hello, my grandson. Welcome to the world!")
* Nau mai ki te ao, e te tamaiti. He mokopuna nui koe. (This emphasizes the grandson's importance. It translates to "Welcome to the world, child. You are a great grandson.")
More poetic:
* He tamaiti hou, he manaakitanga i te ao. Nau mai, e taku mokopuna. (This translates to "A new child, a blessing on the world. Welcome, my grandson.")
Adding a personal touch:
You can also add something specific to your grandson's name or the name of his parents to make it more personal.
Important note: In Māori, gender is not always explicitly marked, so "e taku mokopuna" could refer to a grandson or granddaughter. If you want to be specific, you could say "e taku mokopuna tane" for grandson.
No matter which phrase you choose, it's important to say it with love and warmth.