capricious or whimsical

17:59 May 1, 2017 

You do have a point. I was merely trying to nudge the discussion in another direction. It used to be said that kids crying in the supermarket just want the attention of everyone else around, so their parents would give in and buy them whatever they wanted. That's actually quite "devious" and doesn't happen by chance either :)

Since you work in the DE-EN language pair, I'd like to add that in German, the first would be an example of being "trotzig" (defiant), whereas the second would mean "leicht eingeschnappt" (easily offended). In the US, you may also choose wuss, wimp, butthurt or snowflake for the latter, but this is certain to draw the ire of at least some of the parents. I thought about "balky," as Barbara suggested, but I'm not sure this really covers all bases (e.g., throwing a fit). I guess "touchy" is all that's left to agree to, as you did.

Best wishes and a have a nice evening, all of you

17:24 May 1, 2017 

whether 9-year olds will readily understand "impulsive"

11:38 May 1, 2017 

I'd go with impulsive. First thought of "defiant," since that fits to the first part of the explanation.
http://www.quickanddirtytips.com/parenting/behavior/8-strate...

Not sure you can call that "moody," since he is acting in the same way every time, as I understand the asker - which means he is not "unpredictable" or "unstable." Quite the opposite: If he doesn't get what he wants, you can expect yet another embarassing situation at the cash counter (just had this a few days ago!).

PS: Maybe even "contrary."

09:11 May 1, 2017 

I am not sure I would use either of your suggestions when talking to nine-year-olds.
Capricious strikes me somehow as too grown-up, and calling a boy whimsical sounds (to me) as if you are not taking him seriously.
It does NOT mean ´someone who acts on a whim´
whim: a sudden desire or change of mind, especially one that is unusual or unexplained.

00:42 May 1, 2017 

Perhaps "balky" might be an adjective describing the boy's behaviour

00:24 May 1, 2017 

It´s an informal text to describe the characteristics of this boy for some students who are 9 years old. it´s simple writing.

00:18 May 1, 2017 

Everything depends on the register of your text. It's really impossible here to tell you the best adjective to use as we have no idea of the style of your text. Have a look at some of the synonyms I have posted.