SPOILER ALERT (disclaimer: not accurate bec. It's been a long time since I read it)
But in the novel it's not that the empress harbor negative thoughts about him. The thing that made their relationship like that has something to do with the event in the palace before where they met some nobels, and you know nobels in this kind of manhwa have those I'll intent and greedy thoughts and as the empress she knows how to handle this people like she probably has awareness to this kinds of people but still needs to interact with them but Cedric happen to read his mother's thoughts and blurted out so he kinda exposed her.
I don't think the empress doesn't like her son. From her actions, it's more like she felt guilty and regretful of something that happens in the past and now she doesn't know how to fix it. As she said, Cedric was normal until upto a certain age and she blame herself for it.
So possibly, Cedric might have seen negative thoughts from the empress in the past which made her afraid of him for awhile and by the time she managed to organize her thoughts, things might have turned out so badly that she doesn't know how to fix anymore. Hence, she's jealous of FL who can become close to Cedric
Did the empress not like Cedric? Like he can read the negative thoughts of others. So their relationship is this bad because she harbored negative thoughts towards him?
Also, the empress is a bit childish but maybe its getting enhanced by the spell cast on her.