"The cake is for the children."
Translation:העוגה היא בשביל הילדים.
Why is the copula היא obligatory here? The only notes section I found that talked about it was Adjectives 1 and it didn't really explain when it was or wasn't necessary. Would העוגה בשביל הילדים translate to "the cake for the children" or would that be העוגה הבשביל הילדים?
The translation "העוגה בשביל הילדים" is correct (you do not add definiteness prefix to conjunctions in Hebrew). "היא" is used in this context to indicate that this is a nominal sentence, but is completely optional.
I used לילדים instead of בשביל ה ילדים and it said i was missing the היא. Is he not optional with ל?
Yes. If anything, it would be "העוגה הזאת היא בשביל הילדים". The meaning is different: "this cake is for the children".