OK I am going to give you step by step instructions on producing the perfect baby without cheats or mods. it is a 4 generation project without using a premade to boost the project.
1) Create a magic user with the Child of the Islands trait. It doesn't matter if they started life as a spellcaster or not. But they must have the Child of the Islands trait and must be a spellcaster before they parent the chosen descendant to build your super sim's family line.
2) Get one of the "ghost" elders of the island, summoned by the Child of the Islands trait, of the opposite sex of your caster to join your family and turn it human. Ambrosia is useful here. Have baby with this former ghost. This will net you Sulani Mana and a weak bloodline traits in your kid.
Some useful links
Carl's video guide to Sulani Mana
https://wSulani Manaww.youtube.com/watch?v=nDrH0173sIc&t=8s
Thread in Sulani Mana
https://www.carls-sims-4-guide.com/forum/index.php?topic=30882.0Carl's guide on magical blood lines
https://www.carls-sims-4-guide.com/gamepacks/realmofmagic/magical-bloodline.phpCarl's guide to Ambrosia
https://www.carls-sims-4-guide.com/skills/gourmetcooking/ambrosia.php3) Have the chosen offspring with the Sulani Mana and a weak bloodline traits be a caster and have a daughter. This female sim will have Sulani Mana and a strong magical bloodline traits.
To be more likely to have a baby girl: Eat strawberries and listen to pop music on the radio.
4} Have the female, spellcaster sim with Sulani Mana and a strong magical bloodline traits have a child with Father Winter. DO NOT HAVE FATHER WINTER JOIN YOUR HOUSEHOLD. This will make him no longer Father Winter and your child will not have the Father Winter's Baby trait. The Child will have the following bonus traits: Sulani Mana, ancient magical bloodline and Father Winter's baby traits. The best combo to start a true super sim. Also there is a 50/50 on weather the child will be born a caster but that is super easy to fix once the sim is a teen.
Some useful links on step 4
Carl's video guide on having father winter's baby.
https://www.youtube.com/watch?v=EixLM4IJT58Carl's text guide to having father winter's baby:
https://www.carls-sims-4-guide.com/expansionpacks/seasons/father-winter.phpAnyway that is how to get your start. I recently had a thread on making the perfect super sim. here is the link.
https://www.carls-sims-4-guide.com/forum/index.php?topic=31164.0I wish you luck. If you need more help please ask. And yes once a toddler you can edit any sim. In fact given the forced genetics of the NPC's needed you super sim will likely be ugly if you don't.