The Offspring Seal (Child Seal in the Japanese version) is a Reclassing item introduced in Fire Emblem Fates.
The Offspring Seal will automatically appear in any of the Fates children's inventories if their Paralogue is attempted after Chapter 18 of all routes. It functions similarly to the Master Seal as it automatically promotes them while also scaling their level to match the chapter they are recruited in. The Offspring Seal's usage is locked to the character and cannot be used by other units, even other children. The Offspring Seal can also only be used on a child to advance from their base class (i.e. not classes available from Heart Seals etc.); if a Heart Seal is used on a certain unit while in their base class, and then the user returns that unit to its base class with another Heart Seal, the Offspring Seal will not function anymore on that specific child, but the unit may freely change class after advancing.
The Level of the child character upon using the item varies and are determined by the current level of the parents as well as the current story chapter upon child acquisition. The table for using the Offspring Seal is as follows:
The Offspring Seal is basically an advanced Master Seal, as in addition to promoting the children from their base class, puts them in a promoted class with levels near that of the optimal strength the player/enemy units will have.
Children who use the Offspring Seal will learn all applicable skills upon promoting into that class. For example, if getting a child on Chapter 27, they will already be at max level on base. Upon promoting, they will gain a massive stat increase and learn any skills in that class that they have not learned yet. For example, if the player starts Kiragi's Paralogue when they are at Chapter 27, he will start with his base class skills (since he is already level 20). Upon using the Offspring Seal to promote him into a Sniper, he will be a level 18 Sniper upon promotion and immediately learn Certain Blow and Bowfaire, provided that he did not inherit them from his parents.
Using Offspring Seals are highly recommended as they will bump the children's levels up to match the enemies' strength on that level, save plenty of gold (especially in Conquest, where money must be handled carefully unless one buys the Ghostly Gold DLC), and also increase their weapon ranks upon promotion.