Surely the interpretation of the whole scenario depends on the quality of the relationships within the whole family?
So if the atmosphere is generally strained and awkward, if there's a lot of anger, lots of things they don't talk about, then yes, it could be indicative of a problem.
If the family is generally warm, open and loving, then it's probably just harmless horseplay.
I've noticed that dh has cut back on things like this with dd1 (10), though he might swipe her on the bum as she passed if she'd been jokey/cheeky. He is also inclined to avoid going into the bathroom if she's in the bath, even though she isn't remotely pubescent yet.
I've noticed the change in his behaviour, but I don't think she has. They still have a very warm and loving relationship though, so if she was uncomfortable with him tickling her, she would just shout, 'Get off!' and storm off in a huff.
But in a family with lots of complicated unexpressed emotions, it might not be as straightforward for a girl to draw those boundaries.