A: In general, giving little kisses to other children --on occasion -- is fairly common at this age. Even very young children imitate adults and are curious to "try on" adult gender roles, just for fun. Children shouldn't be shamed about this kind of behavior -- rather, shown how to appropriately express their affection for others.
But where does normal exploration end, and worrisome behavior begin? If your child is hiding her behavior, making other children feel uncomfortable, or avoiding normal childhood activities in order to pursue the "kissing" games, then it's time to ask for professional help. And of course, children who are exposed to adult sexual behavior -- or are being sexually abused -- are more likely to act out, sexually, and should definitely have the support of a professional for evaluation and treatment.
Working with a licensed, highly recommended therapist who is experienced in working with young children is an excellent start. Therapy for children this age should never occur "in a vacuum", and should always include the parents as important members of the process.