Shabbos, Chapter 6, Mishnah 9. The Mishnah deals with items that one may go out with on Shabbos, among them the law concerning children and the sons of kings.
"HaBanim yotz'in bikshorim":
What are these knots? The Gemara explains that this refers to a child who was seized with intense longing for his father, being strongly attached to him. To help him overcome this, they would take the strap of the father's right shoe and tie it onto the child's left shoe, and this remedy helped him overcome his powerful attachment to his father. Therefore one may go out with such a knot on Shabbos, and it is not considered carrying a burden.
"Uvnei melachim bezogin":
The sons of kings, the princes, may go out with zogin - bells woven into their garments. The Tiferes Yisrael explains that these are bells that have no clapper, and therefore make no sound at all. Even though they are an addition of material onto the garment, they are considered beautification and adornment, and one may go out with them.
"V'chol adam, ela shedibru chachamim bahoveh":
The Mishnah clarifies that although it used the phrase "sons of kings," the law applies to every person. The Sages spoke of what is common: usually it was the princes who had such things on their garments, while for an ordinary person this was not common - but if he does have them, he too may go out with them.
In summary: In this Mishnah we learned two permitted items for going out on Shabbos - the strap of the father's right shoe tied onto the son's left shoe, which serves as a remedy for his longing, and decorative bells without clappers set into a garment. And we further learned that the phrase "sons of kings" does not limit the permission, for the Sages spoke of what is common, and the permission applies to every person.