YOU SAID:

they are going to go eat lunch

INTO JAPANESE

彼らは昼食を食べに行くつもりです

BACK INTO ENGLISH

They are going to eat lunch

INTO JAPANESE

彼らは昼食を食べるつもりです

BACK INTO ENGLISH

They are going to have lunch

INTO JAPANESE

彼らは昼食をとるつもりです

BACK INTO ENGLISH

They are going to have lunch