Then they get hyper and keep begging for more
I typed 아이에게 초콜릿을 먹이지 마 and it underlined 마 and said it should be 마세요. Is there a reason it has to be formal?
There is no context, so no, it doesn't have to be. It's just a missing option, please flag it.
There are two exercises in this lesson with the same structure. When 아이 is used Duolingo gives 'to the child' as the correct answer. When 개 is used, 'to the dogs' is not accepted.