My kids get to share a couple of hand-me-down tablets for TV watching on car trips (and occasional at-home show-watching when my wife and I are using the living room TV).
I am not planning to get either one of them dedicated devices of their own until they are MUCH older.
My daughter (about to be 6) has been making "paper phones" for the better part of a year (she'll fold up paper into a cellphone, populate a calendar for the month, draw a picture to have inside it like a locket, and make the face look like a dial-pad). Point is, kids already see and get way too much passive exposure to this stuff for me to want them to have a device that they get to claim as their own (making me the bad guy when I take it away) versus a shared family device that they have to ask permission to use at all.
I am planning to get a cheapie desktop PC to share with the kids, though, so that they can hopefully have some of the same formative experiences I did playing around on his original Macintosh.