I typed "Ces" as you did and mine was marked correct also. Then I checked Google and they translate "Ces vacances sont en Juillet" as "These holidays are in July." They also translate the same sentence from English to French using "est" instead of "sont, but when going from French to English both "sont" and "est" are translated as "is." Fascinating..... Is Google wrong?
Interestingly, at least in America, "their" has become a common word to to be used in the singular when the sex of the object is ambiguous, to avoid sexist assumptions. I am not arguing that "their" ought to be correct here, just making an observation that usage has migrated somewhat, as it often does in English.
Agreed Sarah and I hate the fact it's not correct including being flagged as such in Microsoft Word.
The legal alternative... "his/her" or "he/she"... You have that in a document a few times (which you need to these days) and its the opposite of a good pronoun - ugly, disruptive, repetetive and right in your face disrupting the flow.
Can't change soon enough to "their" being legal really!