The occasional kissing behavior you describe between these two preschoolers falls within the realm of age-appropriate behavior. Most child development experts maintain that the majority of preschoolers will play some sex games during this developmental stage. These games, like the kissing that you described, are attempts to imitate and explore adult behavior and roles.
Unless you witness either of these children showing displeasure with these occasional kisses, don't intervene. If you are debating whether the little girl may feel harassed by this (supposing that it is always she who is being chased and kissed) behavior, you might speak with her alone at some point in the day (not after a kissing incident at outdoor play), asking her how she feels about this boy chasing her and kissing her. Without blaming her, ask a few open-ended questions. Observe how she and this little boy behave with each other throughout the day, not just during outdoor play. That will give you a more complete picture of their relationship and how the chasing and kissing fits into that overall relationship. Thanks for writing and for being sensitive to this issue.