Well, I would have said that my daughter, age 7, would take her favorite stuffed elephant, Ellie, and my son, age 5, would take his Thomas trains, but I thought I'd asked them. Their answers were more practical than I had guessed. This is what they said:
My son would take food and drinks (bananas, cereal and milk), a guitar for music, flowers, and corn dogs so that when he gets close to the sun, he could cook them and eat them. He also stated he would "bring back the moon" from his trip for us.
My daughter wanted to bring cinamon bread, peanut butter and jelly, Fruit Loops and water. She wanted to bring pajamas and a flashlight, too. She also felt she could use a "code reader" to help her figure out any secret codes she might encounter.
Neither said toys...go figure!