The Xanth books, by Piers Anthony.
The boy who conjured rotten fruit is Hugo. The girl who improved people's abilities is Ivy. The dragon is the former Gap dragon, named 'Stanley Steamer' by Ivy.
It could be one of several books, but is probably Ivy's effective debut, Dragon on a Pedestal.
When the book begins, the Good Magician Humfrey, and his son Hugo, run into the Gap dragon while filling a vial with water from the Fountain of Youth. Humpfrey tells Hugo to douse the dragon with the water, and Hugo does so but accidentally sprays Humpfrey as well. Humpfrey regresses to the age of a baby, as does the dragon. Queen Irene realizes Princess Ivy has wandered off, and begins a quest to find her daughter. Luckily, Ivy comes across Humfrey's 8-year-old son Hugo, and – due to her unknown talent of enhancement – Hugo temporarily becomes smarter, braver, and stronger when she tells him he is. Ivy also manages to enhance the positive qualities of the Gap Dragon, and names him Stanley Steamer.
In Castle Roogna, Dor accidentally put a forget spell on the Gap Chasm (the huge rift that splits Xanth in two), while trying to escape a horde of harpies and goblins, with the result being that everyone forgot the Gap Chasm existed, with the exception of the people who live near it. In this book, the forget spell is beginning to disintegrate into "forget whorls" spinning off into the nearby forest (due to the Time Of No Magic caused when Bink released the Demon X(A/N)th), causing confusion and memory loss. Ivy ends up walking through a forget whirl and it causes her to forget how to get home.
Near the end of the novel, all the characters join forces against a swarm of wiggles, which threaten the welfare of Xanth by burrowing through anything and everything in their path.