I've heard a lot of people saying they were children when they started to stimulate themselves, so that's not unusual at all.

As for the first two, they should definitely not be encouraged (especially the second!). Where the first is concerned, I accept that some families are more kissy than others and some children more affectionate than others, so perhaps the child is going a bit overboard and understands kissing behaviour as being more acceptable than others see it, or the child sees a lot of prolonged kissing in people around them and is copying, or the child does understand but is pushing their luck. As for the second, I think a child of that age should know better but I can imagine that, especially if the older child is female, this could be a rather turbulent time hormonally (periods can start as early as eight or nine).

I reckon that at that age a child can start to experience more adult attractions, sensations or urges but doesn't know how to express them or knows that expression is considered unsuitable, therefore their only outlet might be to perform those tasks with someone unable to judge and comment on them, or who are easily dominated- such as younger/smaller children or an animal.