(First, please correct the title of this thread, OP.)
Once at WDW and settled in your room -- if you have at least half a day, and will be staying for several days (enough to visit the Magic Kingdom at least twice, as it offers more than twice as many attractions as the others, and way more to do with a young child than any other park), I suggest visiting the Magic Kingdom for the remainder of the day. Like the previous poster, it's a tradition for my family (most, I'm sure) to begin with the Magic Kingdom, as no visit to Disney World feels legitimate or complete until we've seen Cinderella Castle. If you only have a couple of hours and don't want to "waste" a full-day park ticket, then stroll around your resort, visit the pool, and/or consider a table service dinner at the resort, or at Disney Springs. There will also be a schedule of activities at your resort (e.g., movies, poolside games) that should be given to you at check-in.