After the last update, children can learn charisma (adult) skill concurrently with social (children) skill for example.
Do you like that? Or do you think it is better the old way where kids will only learn charisma after maximizing social?

I have zero problem with it. People learn about dealing with people and what is best or not from the time they are born. I am fine with it and have way too much going in my RL to be concerned with things like this.

I never liked that children had separate skills. That's why I made a mod (when I was still playing TS4) that unlocked all adult skills for them and then I never played without it. It's interesting to see that Maxis separated Charisma from Social skill. Did they do the same with Mischief?

Yes, children can learn all adults skill now. The children skill is still there tho.

I'll like it better now that children can actually learn to sing and play instruments, it was frustrating to have my child sing or play violin all day every day and never get any better.
((please tell me this is the case, I've been playing with infants this whole time and haven't gotten to child stage))

ETA: I also like the idea of the child skills still being there, it encourages you to round out their development and, for example, NOT have them sing or play violin all day every day. The fact that you are still going to want to get those fundamentals up means that just like in real life your child will need to branch out and focus on learning other skills they may never use when they grow up, like logic! (hA, gottem)

I don't think I mind, but does it render the child equivalent of the skill useless? Before it would give them a boost once they entered their teenage years, what happens now? Is it the same?

Is that all skills, including Flower Arranging, Salvadorato Lore, etc?

No, only some skills. The ones they were able to learn after reaching max level on children skill.