My children are the most important things in my life.
Is this is the correct idiomatic expression because my children definitely are not things and I wonder whether I should say thing instead of things.
Yes, that's the idiomatic way to say it. Indeed, those exact words or words very close to them are very commonly said.
True, "things" is normally used to refer to inanimate objects, and your children are (hopefully) not inanimate.
But the trick here is, if you used a word that was more specific, like if you said, "My children are the most important people in my life", then someone might legitimately wonder if you meant or implied that there were things other than people that were more important to you than your children. Like your children are more important than your co-workers or your brothers and sisters, but they're not more important than money or popularity or your new car.
So in such contexts, we say "things" to be more general.
In some contexts it might be ambiguous. Like if you said, "Name 3 things that are in that room", would you expect a person who is in the room to be included in the list, or are you thinking of only inanimate objects? It wouldn't be clear without more information.