@ratchie Another thing you could try is having your Sims drink Emotion Potions, it's a reward you can buy and they're cheap too, they give your Sim a +99 boost for whatever emotion the potion is. I've used these before to override unwanted moods that are too strong to overcome normally, so hopefully these will help you too.
I think children can craft these on the Chemistry Set too although I'm not sure on this as I haven't played with families for a long time, but I do know there is a way to make them (Just not sure if I'm remembering the right way to do it), might be worth checking out.