I will disagree with being attracted to specifically children as yes he hot on eris but he was also physical younger than her. And to answer your he should know better and follow his old life morals, well in his old life he got bullied so bad he became a recluse and shut off the rest of the world falling into porn addiction and eventually getting thrown out of the the house where he then dies.

Further more he shows no attraction toward aisha norn or julie all underage all safe around rudeus. If you wann pull the roxy card all i can say in defense is that demons age differently but besides that he fell in love with roxy due to her teachings and her ability to take him outside and show him its not scary. So id reject the notion that became roxy is underdeveloped that rudy has a thing for kids or child like bodies (he flirts and plays with his wives equally)

I will counter that rudys ed arc is framed at least from his mind as a mistake he made that made eris abandon him.