A couple people pointed out last week that Jekyll refers to "our" mind--the one shared by himself and Hyde. Jekyll doesn't see Hyde as some foreign devil come to taint his soul. He went into his experiments with the intention of separating out the unsavory parts of himself. He is painfully aware that Hyde comes from within, and he entertains no fantasies about his own purity or saintliness.
And so there's no reason for him to look down his nose at Hyde. Hyde deserves respect and consideration and, like any personal demon, has to be let out from time to time, or else . . . bad things will happen. Jekyll has no intention of finding out what those "bad things" are, although if Hyde insists upon acting like a child, he might not have a choice.