One of the modifiers a spell can gain is children, this just means the spell is able to cast other spells. There are a few ways for this to occur based on the type of spell. If you want to create a fireball, you can create a fireball projectile, when this projectile hits a target, it will create its child summoning an explosion. Other examples of this function is a stationary tick spell that summons projectiles are the nearest target creating a turret.

Using this child feature, spells are able to be combined in the shop. While a spell is equipped, the player can drag spells into slots depicted the type of child, this attaches the spell onto the other.