It really depends on how extensive it is. Unlike apparently most people, I don't mind buying content for content (probably because my other main hobby is CG art with Daz Studio, and purchasing add-ons for add-ons is normal and in some cases expected.) However, there would have to be a certain "bare minimum" of features, and it would have to logically incorporate those features into the existing power/perk structure.
Most importantly, if I'm buying extra magic features for kids, I want more than just a couple animations and entertaining but otherwise idle interactions for toddlers and children. I want kids to be casting spells and developing their spellcaster xp. I'm perfectly okay with kids not actually reaching the second stage of spellcrafter or getting perk points, but a kid spellcrafter who is actively using magic should get some sort of leg up when they turn teen.
Kid spells should either be lesser versions of existing spells (Delicioso that only summons quick meals, for example) or there should be a corresponding adult version added (toddler accidental levitation of toys, kids summoning toys that float across a room, adults summoning things that float across the house, or pre-chosen items that can be summoned from a different lot.) I really don't like the idea of "kid only" spells that don't have a corresponding adult ability, such as homework spells. That just doesn't make sense to me with any version of magic I've ever dealt with (and I don't care what was done previously in the sims. I would have thought the same thing then.) I realize that not every adult spell can have a corresponding kid spell, but kids don't need as many spells as adults - perhaps two or three per school.
Regarding potions, I'm less demanding. Kids should be able to help, but shouldn't be able to brew potions alone, and they should get animations to reflect that.
Oh, and kids can't get cursed, but they can have spells fail... a lot.
Now, in terms of CAS or Build Buy - that can be whatever. Kids and toddlers getting some furniture and clothes suitable for Realm of Magic would be great. (Just let the CAS stuff be more colorful - especially for toddlers.)
I'm also not sure how this would expand for Vampires, but I'd want something similar for any of the occults. If I'm paying for an add on, then there had better be stuff that's really being added on, and it needs to fit in a logical progression of toddler-child-teen/adult development of the occult ability.
