(The question has already been "answered" in the comment above, but here is another one from me for the thread's sake.)
Yes, your understanding is indeed correct.
「[五千円]{ごせんえん}で[一万円]{いちまんえん}の[物]{もの}が[買]{か}えないってことくらい、まる[子]{こ}にだって[分]{わ}かる。」
「~~くらい」, in this usage, expresses how minimal, trivial, slight, weak, easy, etc. something is. That "something" in this case is the fact that you cannot buy a thing worth 10,000 yen with 5000 yen.
The speaker's estimate of the triviality of the object is further emphasized by the 「だって」 part that follows. To create this extra emphasis, we often combine 「~~くらい」 with:
「にも」、「でも」、「さえ」、「だって」, etc.
Free original example sentences:
「タバコくらい[自分]{じぶん}の[金]{かね}で[買]{か}えよ!」 "Something as (cheap) as cigarettes, you should buy with your own money!"
「こんな[問題]{もんだい}くらい、[小学生]{しょうがくせい}でも[答]{こた}えられる。」 "A question as easy as this one, even a grammar school kid could answer!"
Finally, a quote from 'Evangelion':
「[人類]{じんるい}を[守]{まも}るくらい、[私一人]{わたしひとり}で[十分]{じゅうぶん}よ!」 "A task as trivial as saving mankind, I could handle all by myself!"