Oh, China absolutely wants to control/exert influence in NK, I just don't see them being at all interested in military takeover of the country -- except perhaps if it falls into the "wrong" hands, in which case they might invade to install a compliant regime and then GTFO (see also: Korean War).
You remember Kim's brother VXed in Malaysia? He was China's bid on NK throne. China was trying to launch a coup in NK.
China been trying to seize power in NK for decades.
NK-China relations are complicated ones. China wants NK, but is afraid of losing NK if their machinations flop, and push the NK towards the West.