The cool thing about talking to kids is that they don't care how bad you are at the language.

My niece Tohi is Korean, and she talks to me like I'm Korean.

And she doesn't care when I can't even form a half-coherent response.

When I meet her next week, in Jeju, I'm hoping to at least be able to form a handful of Korean sentences, though.

I also thought it might help to provide you with some cool phrases in Japanese for chatting with kids.

Stuff You Can Say to Japanese Kids

(Note about kana: I've finally ditched romaji for these lessons. If you can't read hiragana and katakana yet, please go learn them before studying anything else ^_^. It will make your life much easier. Hardcore students can learn them in 1-3 days. Most will probably take 1-3 weeks to really get them mastered.)

何才？
なんさい？
How old are you?

You can use this with adults, too.

In a more formal situation, you might say:

何歳ですか。
なんさい　です　か。
How old are you?

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❈　才　ＶＳ　歳　❈

Maybe you're just now noticing this, but they use two different kanji for the さい in 何歳／何才（なんさい）.

I have no idea why.

*Looks up why.*

This page's explanation says that the correct kanji for "age" is 歳, and the writer claims that 才 became an acceptable substitution over time, simply because it's easier to write.

Specifically, he said:

１　日常的に「何歳」と記録する時など、
２　「歳」という字が画数が多いため
３　「才」の字で代用するのを少しづつ、
４　許容するようになってきました。
１　にちじょうてき　に　「なんさい」　と　きろく　する　とき　など、
２　「さい」という　じ　が　かくすう　が　おおい　ため
３　「さい」　の　じ　で　だいよう　する　の　を　すこしづつ、
４　きょよう　する　ように　なってきました。
Since the kanji 歳 has a high number of strokes, when recording age in everyday situations, the kanji 才 gradually came to be used and accepted as a substitute.
Literally, Line #1: "everyday + に + 何歳 + と + recording + do + time + and so forth,"
Literally, Line #2: "歳 + という + character + が + stroke count + が + many + sake; result; because"
Literally, Line #3: "才 + の + character + で + substitution + do + のを + little by little,"
Literally, Line #4: "permission + do + so that + became-came"
Note: Put simply, the "incorrect" character is OK because it's easy to write.

It's so hard for me to insert a big sentence like this and not try to explain every single little thing that's happening.

For instance, I didn't actually know this word 画数（かくすう / stroke count）until just now. I guess I've never needed to discuss the number of strokes in a character in Japanese before.

Also, this ため might throw some people off. The first ため you learn in Japanese texts is the one that means "in order to:"

１　あなたがテストに合格するために、
２　スタッフ全員で協力します！
１　あなた　が　てすと　に　ごうかく　する　ため　に
２　スタッフ　ぜんいん　で　きょうりょく　します！
All of our staff members will help you to pass the test!
Literally, Line #1: "you + が + test + に + passing + do + ために"
Literally, Line #2: "staff + all members + で + cooperate; collaborate + do"

Note: I pulled this sentence from this flower shop's webpage. Apparently they give all new employees a test on flower types and flower arrangements... and all of the employees help the newbies to pass.

Also, the author of that sentence on 何歳 ＶＳ 何才, above, said 少しづつ（すこしづつ）, but I usually see this written as 少しずつ（すこしずつ）, "little by little."

I read an article online saying that the pronunciations of づ (dzu) and ず (zu) are different, but then I also read an article that said native speakers actually just say ず all the time. I can see how people might find these spellings confusing.

This problem is referred to as 四つ仮名（よつがな）, which is a technical term referring to the difference in pronunciation (or lack thereof) between (1) ず (zu) and づ (dzu) and (2) じ (ji)、and ぢ (dzi).

Theoretically, they should be different sounds, but a lot of native speakers pronounce them the same. This Japanese Wikipedia article talks about it a lot. The English version, meh, not so much.

Here's the short version: Nobody cares.

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By the way, you'll also occasionally hear people use the word いくつ, meaning "how many," when asking about age:

彼氏いくつ？
かれし　いくつ？
How old is your boyfriend?
Literally: "boyfriend + how many?"

Note: That literal translation might make you wonder why this doesn't mean "How many boyfriends do you have?" We don't have that problem, because talking about people would call for the "people counter," 人（にん）.

彼氏何人いるの？！
かれし　なんにん　いる　の？！
How many boyfriends do you have?!
Literally: "boyfriend + what-people + is + の？"

Focus, Niko, you fool.

Sorry.

We're supposed to be talking about phrases to use with kids.

But I went and got all ADD on Japanese.

Moving on...

大きくなったね。
おおきく　なった　ね。
You've gotten big.
Literally: "big + became + ね"

学校楽しい？
がっこう　たのしい？
Do you like school? // Is school fun?
Literally: "school + fun?"

おもちゃ買ってあげる。
おもちゃ　かって　あげる。
I'll buy you a toy. // I'll buy you toys.
Literally: "toy + buy and + give."

Bonus Phrases

"四つ仮名"？なにそれ？
よつがな？　なにそれ？
"Yotsugana?" What's that?
Note:"Yotsugana" are these four kana sounds: じ, ぢ, ず, づ. I wouldn't expect Japanese people to know this word.