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It is somewhat within the range of normal development in many children, especially between the ages of 4-8 years of age. It also depends on the nature of the touching and kissing. Many children model behaviors that they see on television and witness as observers. It is quite possible this was very innocent and it ceased...a very critical piece of information.
Truthfully, and I have see this a lot with children who act out sexually and have clinical issues. They are often fairly blatant about the sexual behavior at the ages you describe. There is often genital contact and more...The behaviors tend to escalate, not decrease. I am not seeing that here.
All in all, I think you can relax. If this was a limited and focused event, as it seems to be, I would mark it as an odd glitch in development and leave it at that. Take heart. I see little to worry about here. Steven