i remember that the Seme is a demon i think, hes huge and he can hide in the Uke shadow, he think he's ugly but he's adorable and the Uke is a middle age man, but he's very seductive and like to sleep around, this is a manga.
This is what you're looking for i think http://www.mangago.me/read-manga/the_monster_and_the_beast/
its like the eminence in shadow knock off, but much worse, im 2 chapters in, the author really dont care to build out the relationship between MC and the children, its just skip to him ordering someone to save them and him sending letter, everything is so fast pace, and just not properly shown.
I did agree at first, but he builds a relationship with them later on which makes the story nice. If you continue to read and still dislike it that’s understandable, but give it more of a chance before making an opinion.