You can also omit the 里：他们在厨房做蛋糕。
Zai and li -why both are required. Don't they mean the same
As far as I know 在 can be translated as "to be" when it comes to locations. 里 means inside. You could say: 我在房间 and 我在房间里。but the 在 is required. You can't omit it.
Sorry for the late reply.