Preparing food on Yom Kippur:
For after the fast: It is forbidden to prepare food on Yom Kippur for after the fast. This applies even after Mincha of Yom Kippur.
For a child: One may prepare food for a child on Yom Kippur if the food is not edible otherwise.
From the letter of the law it is permitted to prepare food for after the fast, beginning from after the time of Mincha, with exception to when Yom Kippur falls on Shabbos. The reason for why it is forbidden to do so before Mincha is because when preparing before Mincha we suspect that during the preparation one may come to forget it is a fast and will eat from the food. [611/5-7] Nevertheless, the custom is to forbid preparations even after Mincha due to suspicion that one may come to prepare food even before Mincha. [ibid]
 The difference between Chameitz on Pesach and food on Yom Kippur: This is opposed to the law regarding Chameitz on Pesach in which case it is forbidden to touch due that one may come to eat it. The reason why by Chameitz we are more stringent is because since one is already involved in the act of eating which is permitted on Pesach, one may come to also eat that which is forbidden. However on Yom Kippur being that it is forbidden to eat at all, therefore there is no need to suspect that touching will lead one to eat.
Q. Why is the food on Yom Kippur not considered Muktzah being that it has no use? Seemingly it is similar to all inedible foods which have a status of Muktzah, as is the law regarding Chameitz on Yom Tov of Pesach.
A. On Pesach Chameitz is forbidden even in pleasure, thus it literally has no use. Furthermore Chameitz is forbidden to everyone, even children. However on Yom Kippur food is permitted to be eaten by those under the age of Mitzvahs. It thus has an edible use on Yom Kippur itself.
Q. Nevertheless why shouldn’t the food be still be considered Muktzah Machmas Issur?
A. Perhaps only an item which is a general Issur for all can be considered MM”I, however if it remains permitted for some (like children) then it’s not Muktzah. However perhaps children are considered an exception due to danger, and thus it is anyways considered forbidden for all, and thus should at least be MM”I. ע“וצ