in this panel we see that sanji is able to use CoO in nami's body - haki is literally will and spirit. it does not change even if a user moves from body to body.
its a property of a person's spirit and not a property of a person's body. momo has no formal training in CoO and CoA. so by simply gaining a grown up body, his spirit wont be trained. he'll still wont be able to use CoO and CoA so harming kaido is out of the realm.
Momotaro tale involes him capturing the oni and not killing them.
Instead look at the susanoo legend. The god susanoo - uses a sword called "Ame no Habakiri" and cuts up the 8 headed serpant monster orochi and then reunites with his sister, Amaterasu.
This is the purpose of Oda giving Momo the ame no habakiri. He will kill Orochi (8 heads in total - 1 cut by kaido, 6 cut by the scabbards). And i dont think you need haki to cut up fodder like orochi. and then he will reunite with hiyori. But it will suit the tale perfectly. I think momo will take luffy up and have some dragon vs dragon fight with kaido. he'll hold onigashima from falling down and then will slay orochi using the ame no habakiri. he isnt going to kill kaido - because 1 - he has no haki, 2- it is not his destiny to kill kaido, its his destiny to lead the rebellion and wano of the future, 3 - its his destiny to kill orochi since he was gifted the blade which was used to kill orochi in the mythology.