Three months is the traditional age for the baby blessing or "Wiccaning," any age is fine, including an adult child. You will find this very familiar. It is the ritual that the story Sleeping Beauty distorted when they made the High Priestess of the Coven into an Evil Faerie Witch. We've done it in my family for years, and it seems to be radiating out into the world now.
The child's group gathers, usually bringing a pot luck. The group should be small, only the closest relatives and Goddessparents, not more than 13. (Take liberties with this. If you want to invite 200 people, just remember that the kid will get pretty cranky after a while and you'll bore everyone else to tears).
If you want to get really cute, you can mark the quarters with stuffed animals or toys.
I would recommend that gifts for the baby be simple and symbolic. They should reflect the Gift that is being given during the ritual. The mother certainly doesn’t need any more clutter right now. Save the fancier stuff for next year, when the baby is old enough to appreciate it.