WARNING: The following contains spoilers for One Piece Chapter #1021, "Demonio," by Eiichiro Oda, Stephen Paul and Vanessa Satone, available in English via Manga Plus and Viz Media.
Little Momonosuke may not stay little for much longer if his new request in One Piece Chapter #1021 is honored. The son of Oden has asked Shinobu to use her Ripe-Ripe Jutsu to make him an adult so he might be able to transform into a large dragon and help Luffy. So far, Shinobu has denied the request because the effects are irreversible, but Momo is determined and stands his ground on the matter.
Momonosuke was eight when he and his retainers were sent 20 years into the future to protect him as the heir to Oden's title. Had this time travel not taken place, he would be 28 and likely more capable as a fighter. He is still a child though, and as such there's little he can do with his short stature. Others are constantly protecting him because he is weaker than the rest of the group. On top of that, his dragon form is also small, most likely reflecting his child stature.
Poor Momonosuke has watched as the most important people in his life were cut down by Kaido, seeing everyone pushing themselves to their limits to free Wano from the villain's rule. While he played a major role in the raid by announcing Luffy's survival and boosting the Alliance fighters' morale, he feels he can do more. Momonosuke has the ability to turn into a dragon like Kaido so if he were larger, perhaps he could go head to head against him. While size is only a part of what makes Kaido powerful, it does pose a problem for the other fighters because he towers over them.
If Momonosuke were an adult, not only would he not need everyone to constantly protect him, but his dragon form could increase in size and possibly power. Momonosuke's childhood has been stolen by Kaido and the Beast Pirates, and he can never get it back. All he can do now is focus on finding the best course of action for Wano, and becoming an adult is the next logical step in his mind. The country will need a capable leader even after the battle, and while Hiyori is an adult, Momonosuke is the one who will lead. Being an adult in One Piece is not only about doing what's best for the fight at hand, but also for the country's future.