"I will transform into a young child."
At first nothing smiles, and you frown and start to get back to work in the basement, then suddenly you notice things start to get larger. Your pants and underwear fall off of you. You continue to shrink, and your shirt falls down around your waist. Then the changes stop.
You climb out of your oversize shirt, and look down at your naked body. You couldn't be older than six! The journal had worked! You quickly ran back over to it and picked it up.
Could it be used to transform others? Maybe you could check, or maybe you should find some fitting clothes first, or write some up, or arrange it so you don't need clothing. You put the pencil to the paper and start to write....