"Le chapeau des garçons est grand."
There is a French style of singular/plural that doesn't translate very well to literal English. For instance, "our hearts" (hearts is plural, because there's more than one of us) is "notre coeur" (NOT "nos coeurs" -- the plural form).
In other words, in French, you say "the performance touched our heart" (singular!). It sounds crazy in English, but to use the plural is equally crazy-sounding in French.
It's hard to get used to, and the chapeau sentence does "feel" incorrect. (I'm still suspicious of it, at any rate.)
I translated it as the boys' hat is big . Unless you mean a group of boys are sharing one big hat this doesn't make sense in English. However that is what the French words mean.
Duo accepted it as correct.
On any of the straight translation exercises that Duo gives I wouldn't have used such a strange construction.
I can see I'm not the only one having a little difficulty with French/English plural usage.