Having children is a multiplayer feature. Got to have another player of either the same race, or human/saiyan couple, and press B to open the menu. If playing singleplayer, the only way is to spawn one with a console command, and it’ll be randomized rather than based on your character.
/jfc spawn child [childname]
Again, child will be randomized, might not even be the same race as you. Only way to get a child with an appearance based on your own character is to do so with another player.