Fatwa Translation summary:
Question: My 12 year old asked me a very strange question and I couldn't answer it. Is this question wrong or not? And what should I do in these kinds of situations. He asked me, "In jannah, is it possible to marry cartoon characters?"
Answer: Fatwa number ... clarifies that whatever a person wishes for in jannah, he will get. As for wishing for something imaginary, if it's possible and there's nothing similar to it in the world, then Allah will make it real, like clouds that rain full breasted maidens, or similarly, marrying a cartoon character because Allah can breath a soul into her. [Of course, he assumes it'd have to be a her.
]
What is required is that a servant should ask his Lord for jannah, and not go into too much detail. Because if he gets to enter jannah he will get whatever he wants.
And what you could do is guide your son towards what is even better than that in jannah, like marrying hoor 3ein, because they are better than the cartoon characters he's imagining, and call his attention to the fact that he'll only get there through good deeds.