Huh... I genuinely didn't know children could die in Sims 4. Mostly because random deaths never happened to them in my own game, and any time need based risks of death creep up the game notifies that they'll be taken away if I don't tend to them properly. That alone, and how S4 is very happy-go-lucky overall, contributed greatly to why I just assumed they couldn't die. Even just from playing games more violent in nature, such as Skyrim, you need mods to remove child immortality.
I'm glad to hear they're not immortal, though. Not for twisted reasons, but for the sake of realism.