Generally speaking abuse is committed either by a child old enough to understand the effects their actions can have on another. Children's brain chemistry is not fully structured in a way to be able to make decisions based on long term effects. The longest they generally think of is "what will i get for christmas/birthday/report card?" or be concerned "uh oh mom and dad know i lied - now what will they do?" Beyond those kinds of things, they don't really consider. You may have known it to be "wrong" - but you were experimenting. It's something a lot of kids do innocently, trying to figure out why girls are different than boys and how each part works. It's what people have termed "playing doctor". It's not encouraged, but it's innocent - unless sex play comes into it, then questions start arising as to how the child learned about that. The child still is not to blame though. The adults are. Generally, it is encouraged for parents to sit down with their children and explain all these things and answer any questions when they start exhibiting curiosity, to prevent these kinds of problematic emotions later, as well as problems between parents and confusion between the children.

The kissing - that was sexual abuse and not your fault.