Sim hates children, but it's not working out the way I was expecting

Yeah, the hates children trait doesn't really work on the sim's own children. The only differences are that if a sim is pregnant they have a constant tense moodlet instead of a happy moodlet, they can smash the dollhouse when not angry, and they have a mean interaction to "crush dreams" on toddlers. You can try making her use that and other mean socials on the toddler to bring their relationship down.