1. Head over to buu’s house from the time rift around conton city.
2. Feed buu enough food until he gives birth to a child.
3. Feed the child food, it will go on a walk.
4. Do around 3 quests and the child will have returned.
5. Repeat the steps 3 & 4 until you are given the option of guard or calm down.
6. Picking guard will cause the child buu to change colour unlocking the trophy.