There are other options for hiding the kid than the purely mundane - A Permanent Mage's Private Sanctum comes to mind (or several - given enough time and EXP, you could cover a sizeable estate); A Screen (Sor/Wiz 8, Trickery 7) would do the job, but it only lasts 1 day, and can't be made permanent (although you might be able to make a minimally intelligent item that can cast it once per day, and does so every day at the same time with the same illusion on the same criteria so that the place is always covered (24 hour duration)); an Amulet of Proof against Detection and Location will help (but only makes scrying unreliable, doesn't stop it - good for day trips, but not much else); a permanent Prismatic Sphere/lots of Permanent Prismatic Walls would make a good bedroom for the kid (hard to get past THAT barrier, if you can pull it off).
However, if you are going for a magic item/ring/golem nanny, make it intelligent with:
Resiliant Spehere (Sor/Wiz 4) (Quickened (+4), if feasable): For a quick way to keep the kid safe if trouble pops up (only a relative handful of ways to defeat it). (Good candidate for a Dedicated power - protect young children/babies)
Dimension Door (Sor/Wiz/Travel 4) (Quickened (+4), if feasable): for getting the kid out of trouble when Resiliant Sphere just doesn't cut it. (Good candidate for a Dedicated power - protect young children/babies)
Secure Shelter (Brd 4, Sor/Wiz 4) or Tiny Hut (Brd 3, Sor/Wiz 3): So the kid always has shelter available (may skip, as Resiliant Sphere should be able to protect quite well... but it's nice to have on hand so the parent can care for the kid overnight).
Mage Hand (Brd 0, Sor/Wiz 0), Unseen Servant (Brd 1, Sor/Wiz 1), or Telekinesis (Sor/Wiz 5): Handle mundane chores, like clothing changes, or pushing a baby carriage; also permits the item to move around and find the kid if they get separated somehow.
Create Food and Water (Cleric 3): Make sure that the kid always has something to eat
Prestidigitation (Brd 0, Sor/Wiz 0): Mostly for the Cleaning aspect and Flavoring aspects (Created food and water is rather bland) - a dirty diaper qualifies as "soiled", right?
Possibly Floating Disk (Sor/Wiz 1): Instant Baby Carriage over virtually any terrain.
Mending (Brd 0, Clr 0, Drd 0, Sor/Wiz 0): Don't want the kids clothes having holes, do you?
Possibly Message (Brd 0, Sor/Wiz 0): so the item can talk with the kid, sing quietly to it, et cetera, at will, or call for help from the party, if the intelligence on the item isn't high enough that it can do that kind of thing anyway.
Edit: A few other nifties would be the various Hand spells - Helping Hand (summon the party, Clr 3), Interposing Hand (keeping enemies at bay, Sor/Wiz 5) or Forceful Hand (keeping enemies at bay, Sor/Wiz 7), and Invisibility (Brd 2, Sor/Wiz 2, Trickery 2, for keeping the kid out of sight).
That said, however, he does still need a parent - the above just handles the mundane aspects (feeding, changing, protection, et cetera). Kids need someone to run to when they are scared, when they need advice, someone to talk to, et cetera.