My experience with the cow plant hunger cycle is pretty messed up too -- it seems that they refuse food more often than not, but then the next time I look at them, they've already starved to death and there's just a 'skeleton.' I'm not sure if it's really working as intended.
As for worrying about them eating Sims, there's really not much need for that in TS4. The first time a Sim 'takes the cake,' they get spat out after being drained of emotion. So that's really not terribly dangerous. Only if they try to grab the cake a second time when they're already 'drained' will they actually be fully eaten and die. It's way safer than it ever was in earlier incarnations.