Previously "chlopcy" meant boys, then boy and now it's boys again. Can somebody tell me why?
The singular/plural confusion may be for feminine and neuter nouns, but for sure not here - "chłopcy" will never be a singular form.
If it were "My boys see a cat", wouldn't it be "Moi chłopcy widzą kot" since the verb 'widzić' is accusative?
Kot is a masculine animate noun, which means its accusative form is the same as the genitive. In this case: kota.
As it is something the boys are not doing in generall it even has to be the progressive aspect. Without ing it is something the boys do regularly or in general, which is a bit strange.