[MMM]

There are plenty of indoor things to do in Umeda, Osaka. Your son would enjoy the Kiddyland store and the wacky water fountain below it. It is accessible from any of the Osaka stations without going outside, but Hankyu is the closest.

I think it is Shin-Imamiya station (Osaka Loop line) that is in front of the train museum. I think it is a short hop across the street and is a good place to kill a couple hours. I am not sure in Osaka where your wedding is.