키스 is equivalent to "kiss" in English, while 뽀뽀 has the nuance of baby talk. So in most serious adult contexts, 키스 is preferred. When dealing with children, 뽀뽀 is the right word. As for the phrasing, 키스를 해주세요 is fine, but 키스해 주세요 is probably more common. One is the short form (키스하다) and the other the long forms (키스를 하다) on the same idea. There are many verbs that can work this way: 말을 하다/말하다, 실수를 하다/실수 하다, 고백을 하다/고배하다, 화를 내다/화내다, etc. The short form is the simplest and usually preferred way in conversation, but when you need to add more detail (an adjective or adverb), the long form is often more flexible and natural. For example: 1. 실수했다 = I messed up. (simple expression) 2. 큰 실수를 했다 = I made a big mistake. (when the stem 실수 is modified by 큰, it becomes 실수(를)). Similarly, 키스해 주세요 based on 키스하다 is short and natural. When you add other things, 키스를 하다 might be a better choice. For example, 영화의 마지막 장면에서 두 주인공은 열정적인 키스를 했다.