I think Jij bent een kind should be accepted on the "Type what you hear" there is NO WAY we can determine from a robot speaker whether they are emphasizing you vs me or someone else, or the casual "you"... so both should be accepted. As of this writing... they don't accept "jij bent een kind."
I understand the distinction between drawing a difference (you are BUT I am not) but with ONLY "You are a child" written--there is NO WAY to know if it's casual, or drawing that distinction. With all due respect, and I LOVE learning Dutch (my levels should show that) I think Jij should be acceptable given all that is shown.
No. If you would translate 'je bent een kind' as 'you are being a child' you would be implying a totally different meaning to the sentence, namely an action (behaving like a child) instead of a state (being young). I think 'hij drinkt thee' can be translated as 'he drinks tea' or 'he is drinking tea' because in colloquial dutch the distinction between present simple and present continious is not always particularly apparent in a difference in verbs.