It's not grooming, it just feels like it is because of his "child" nickname.
At the beginning, the Lord Mugo who asked for something sweet from younger Jaheon isn't the three Mugos we know, but Lee Muhan, which we found out a little later in the story.
The only reason elder Mugo refers to Jaheon as 'child' is because he is a younger mortal, and elder Mugo is... Elder. So no, not grooming. Although it's starting to turn out that he's going to assault Jaheon while he's drunk

yuck, ew. pls dont bring it like "child" or something like that, makes it really uncomfortable. Why not something like beloved or my treasured soul? yall are grown ass mtfs ╥﹏╥ (hold on, is this considered grooming now that im thinking about it more seriously?)