I would never have expected this level of vitriol about a faulty test item.

[xocet]

16. The word 'decade' does have a more general meaning...

decade

...

3. a group, set, or series of ten.

http://dictionary.reference.com/browse/decade%20?s=t

So, it really would not be lying, but it would be confusing to them I would think.

What is the spirit of the course? How general is it trying to be, and how does that align with the upper elementary grades? Is there any good reason to try to be this general so early? How do children of that age respond when presented with such generality?

Here is an example of the usage that you noted:

French republican calendar

TITLE: calendar (chronology)
SECTION: The French republican calendar

The seven-day week was abandoned, and each 30-day month was divided into three periods of 10 days called décades, the last day of a décade being a rest day. It was also agreed that each day should be divided into decimal parts, but this was not popular in practice and was allowed to fall into disuse.

http://www.britannica.com/EBchecked/topic/154851/decade

All that being said, it doesn't seem like a good idea to be so general without a firm foundation, but I don't know how children react to this material. Good luck!