Childish? I wouldn't say he's "really" like that at all.
If anything his point of view is pretty realistic. He knows his limits and, despite being a lazy bastard, spent years preparing himself to go back to the sea where he lost all he had worked so hard to get in his first attempt of becoming the Pirate King.
In all seriousness, I would say he's more mature than Crocodile.
But of course, who wants to be "serious" and boring 24/7. Moria like any other person needs a break. Acting childish, being surrounded by toys while taking naps, and getting all excited over stuff like new additions to his army it's just a way for him to get rid of all the stress.
Or maybe, you know, Oda didn't want to just plaster drama on Moria. He had to make him attractive to his target audience.