Translation:She will sleep later when she is eating.
grammatically this sentence may be ok, but it really doesn't do anything for teaching the adverb 'nanti' ('later'; ie after a period of time) when compounding it with 'ketika' ('when', 'as', 'at the same time') and a clearly impossible choice between sleeping and eating at the same time. the best i can come up with here is:
'she will sleep later when she has eaten'
but for clarity i'd suggest 'ketika sudah makan' to make it clear she is going to sleep after having already finished eating.
otherwise, as others have suggested, 'she will sleep later after she eats', but then the indonesian sentence needs changing; eg 'dia nanti akan tidur setelah makan'.
I wrote, "Later, she will sleep while eating." However, I was marked wrong. (I thought it might be idiomatic and/or contain a bit of sarcasm. I imagined it being said of a child who was so active or acting up that one could imagine her actually falling asleep while eating.) Should my translation be accepted as correct? I did not report it because I feel uncertain.