Mothers (and perhaps fathers) kissing children may be a human instinct (I haven't seen definitive research on this, though I have read that it may help transfer children's germs to mothers so that mothers can produce anti-bodies in breast milk—this is, however, just a theory last I checked). The romantic kiss, however, is definitely a cultural practice and not a human universal. For more details, including examples from around the world and a statistical look at the issue, see my older answer in:
Hi, you may find /r/AskAnthropology to be a better fit for this question.