𝖶𝗋𝗂𝗍𝗂𝗇𝗀 𝖳𝗂𝗉𝗌 𝖿𝗈𝗋 𝖫𝗈𝗌𝖾𝗋𝗌

𝖳𝗂𝗉𝗌 𝖥𝗈𝗋 𝖶𝗋𝗂𝗍𝗂𝗇𝗀 𝖬𝗈𝗋𝖾 𝖦𝖾𝗇𝗎𝗂𝗇𝖾 𝖢𝗁𝗂𝗅𝖽𝗋𝖾𝗇

First things first, I'm going to list some things that children do that people who don't spend times with kids don't know :)

✎ They can happily watch the same movie/video/song/show/etc over and over again. Like, rewind and watch it again.
✎ They will either talk nonstop or are dead silent and refuse to speak. There is no in between. Alllll ages do this.
✎ Children wear weird clothing if they dress themselves. They don't have a sense of style. Eg, my sister once wore a Chinese Airline Towel as a scarf. They come out of their room and you have to take them back in. Style comes in around 8.
✎ Social conventions where? Kids don't care as much about things like names. Kids are just happy to play with each other, no names required.
✎ Kids don't just laugh all the time.
✎ Unlikely to think their actions through before the age of like 6/7
✎ They have a smaller worldview. So breaking their doll is a big deal to them because they don't have anything to compare it to. This stops around 10.
✎ Kids, especially under 5, are very tactical. They're going to want to taste and touch the thing.
✎ Give the child something to obsess over! From about 2-11/12 children generally have one thing at a time that they absolutely love. It can change, and probably will, but it makes the kid more realistic. It can be really general (lego/barbie/dinosaurs) or specific (a book series/movie/colouring). You don't need to bring it up every single time they speak but just keep it in mind.
✎ Don't forget about the kid! Okay, this is one of my biggest pet peeves in books. Don't introduce a child and then forget about it. Just a casual sentence once and again to show that you haven't forgotten about them. Random example: A: "Should we meet at three?" B: "My babysitter gets out of school at three-thirty. Can we do it then?" And now this parent is shown caring about their kid and I'm not annoyed lol.
✎ Don't make all children carbon copies of each other. Have more than 2 children? Make them different. A lot of the time I see the same basic personality in the children. Some kids are really social and some are really shy, even siblings are different! Give them different hobbies or personality types to differentiate them.
✎ Children aren't dumb. Don't take lack of experience as being dumb. Kids may be wrong but their ideas aren't inherently dumb when you think about them. Let's talk money for a moment, now as an adult, we know that we can't just print more money so people aren't poor because of inflation and stuff. A kid doesn't see it like that. Inflation isn't on their mind just the knowledge that we can print money. See what I mean?
✎ On the same wavelength, kids aren't super smart. Don't let them be uber-intelligent unless their an actual genius (and even then they won't know everything). Don't let them have all the answers all the time.
✎ Give them goals. Kids are people you know? The goals don't need to be life goals, more by-the-minute goals. Like, the goal for 9YearOld Sally is getting a cookie, so what's she gonna do? That's up to you. But still them goals. Also, most kids know what they want to be when they grow up (dancer/police officer/vet) that can influence them (wanting to go to dance lessons or getting a vet set).
✎ Don't make them too cute. Cutesy child characters will become annoying pretty quickly. If they are meant to be likable yet they are sickly sweet all the time they'll come across is boring and annoying.

Let's talk about how kids talk. 
Quick, general statements.

✎ SO MANY QUESTIONS.
✎ Some kids are shy some arent. Depends on the situation/who's around/what the kids like.
✎ As stated, kids aren't inherently dumb but they also don't understand because of lack of context. Their language reflects that.

Also, development is different for some kids. Developing doesn't reflect how smart they are, it's just how it goes (like physical growth). In my experience, firstborns will talk more and earlier than siblings later. 
Top Tip: Watch some YouTube videos (if you can stand them) aimed at the age of the children. This kind of language on those should be used in your fics. If they're young (under 5) find some videos teaching them how to talk on YouTube to see where they should be.