Does she know what she is doing? Yes, she knows what she is doing. She does what she wants and what comes naturally to her. Is she aware of the adult implications of her behavior? Probably not, but then why should she? She doesn't need to know.

She obviously likes you, perhaps even has a crush on you. She is aware that she can behave in any fashion with you, and you will not dismiss her or ignore her. You give her the attention that she desires, unlike most other adult figures in her life.

My advice; enjoy it. Don't actively encourage it, but you can be honest with her and tell her if you reciprocate feelings that she has expressed. If she wants to suck your thumb, enjoy it, if she tells you she misses you, tell her you miss her too, if she rides your legs let her.

The fact of the matter is is she is just doing what comes naturally when a girl likes someone, she is flirting. Let her. Enjoy it. It is very unlikely that, at that age, anything would come of it that would be considered so inappropriate it would warrant extreme measures. She is in charge. If she is in charge, it is very unlikely to go very far. And if it does then, you can always take a step back.

But no matter what happens, trust her, don't go to the parents.

DS