In my opinion, she just doesn't do anything interesting or give me any reason to really care. She's not particularly attached to the PC, we're not really given any reason to feel emotionally attached to her past being told we're meant to be, we have basically zero input on her as an adult character or ability to roleplay around her, she's just kind of there.
I feel like, if you have interacted with her while she was your little girl, you get enough reason to care for her. Especially with events such as her being sick or her interaction with you when you play with her or give her food. The feeling of not being able to role play around her when she grows up is one I can get behind. But I feel that is feeling can be expanded to the rest of the game. Your PC is not really you to begin with. You are not really being able to roleplay. Which I mean, sucks, since this game is DND inspired (what is not these days). But at same time, I will give Observer some credit. You interacting with her as a little girl has a lot of variation and it does have many decisions that do impact dialogue and it does lead her to take one of 2 paths.
The whole thing is just a bit off for side content with a major time skip. Would probably work better if it was the main romance in a story, but when she's your 30th kid and her mother is one of 20 women you regularly hook up with while you regularly wander around screwing demons and ruining lives it just rings a bit false, to me. It's kind of true for all of that content though, it's like it was written for a set PC and anyone different was just completely ignored.
With your last part I somewhat agree. This is one of few porn games where I do feel kind of weird and guilty for promising my love to a lot of women and some men. But I only propose to men when I can live out my fundashi fantasies). But at same time, It does kind of empower you. You are so good at dicking down, that half of the people you fuck just can not get you out of your head. Even the sluties of girl such Cait and Elthara can not help but toot your horn while fucking other people. Hell you are so good, that Brint, the perfect bro and polyamory husband, praises you on how you are compareble/better to Cait, the priestes of sex god, even if you did not touch him. You emit such sexual arua that imagining sex with you is already better then having sex then with most people. And other half give up on fucking anyone else but you (Kiyoko and Brienne as most popular examples). Which is basic as sexual fantasies goes, but why fix what is not broken.
And while it is true that if you do not care about Kiyoko that much, all of those declarations of love can feel kind of empty. But that can be said for a lot of characters in this game. If you do not care about them, you will feel disonence with the game since PC, as we established, is much of a character themselves (somewhat incosistant but what can you do) as anyone else. Not best for roleplaying but it does its job in my opinion.
And I for one love Kiyoko in this game. A classy aristocratic girl that is strict yet cares for you kids children, keeps up her royal image for everyone else but really only opens up to you (heh) and shows her more carnal side. That of natural born mother that is always ready to birth more of your children. And even despite your difference in statues, you both see each other as equals and lovers (especially if your PC is a warrior or comes from slums. ). I especially love taling with her and I find her interaction with my Futa PC one of the sweetest yet entertaining in the game. Her drunken bath scene is so sweet. Especially if you refuse to have sex with her.
And when it comes to Kinu as a child, I am sorry, but she is not compareble to rest of your children. Almost every other child you make or birth just end up being of another stat. Which I think is the best because I, like most men and some women, have impregnation and pregnancy fetish, but caring for children is not really something most of us care for. Which does make me somewhat contradict myself by saying that I enjoy spending time with Kinu at both stages of life. But she is exception.
And her being only child that you can interact with that is not part of other characters scene is nice. Which makes her more unique.
Should have just aged her up a few years or something, so we can see the impact we had, while keeping her a recognisable character instead of someone completely different to the point of effectively being a whole new NPC.
This is where I disagree with you most. Sure she changed, but it makes sense, at least in hime route. She still keeps a lot of her manerism and Is polite as ever. She just acts more mature, but still keeps a lot of her childlike mannerism.
In the end it sucks that we can not go all alabama with her, but I just can not see eye to eye with people. Now excuse me, I have to have sex with one of my many wives.