وعليكم السلام ورحمة الله وبركاته

Yes, Allāh will grant the people of Jannah whatever they desire. Allāh says: {لَهُمْ مَا يَشَاءُونَ فِيهَا وَلَدَيْنَا مَزِيدٌ} "They will have whatever they wish therein, and with Us is more." [50:35] A child's imagination is vast, and they may express their desires in ways that seem unrealistic to adults. Rather than dismissing them, it is better to nurture their love for Jannah while gently guiding them away from admiration of characters like Spider-Man. Know, dear sister, that cartoons, especially if they contain elements that contradict Islamic beliefs or undermine morals and ethics, are impermissible. The Marvel series, in particular, includes characters that distort and negate religious truths, such as those possessing supernatural abilities like bringing the dead back to life or having knowledge of the unseen. Some of these elements fall into the realm of sorcery and witchcraft, while others imitate the attributes of the Creator, which is impermissible. Instead of saying that they can become Spider-Man, which might strengthen their attachment further, a better response would be to tell them, "What Allāh has prepared is far greater than anything we can imagine; it is even better than what we can think of." This response aligns with the statement of Allāh in the ḥadīth Qudsi: ((أعددت لعبادي الصالحين ما لا عين رأت، ولا أذن سمعت، ولا خطر على قلب بشر، فاقرؤوا إن شئتم: {فلا تعلم نفس ما أخفي لهم من قرة أعين})) "I have prepared for My righteous servants that which no eye has seen, no ear has heard, and no human heart has ever conceived. So recite, if you wish: 'No soul knows what joy is kept hidden for them as a reward for what they used to do.'" (32:17) [Bukhāri & Muslim] This way, you nurture his excitement while keeping him away from being attached to these negative influences. May Allāh bless your efforts in guiding your brother towards the love of Allāh and Jannah.