Hmmmm, it's honestly not what Helen does to Percy's finger as much as what Percy does to Helen's finger. Let me show you the exchange here and you can see for yourself. Martha and Percy are playing with paperdolls, and Percy is pretending to give his paperdoll an "operation":
(But Helen is poking her fingers inside his mouth, to feel his tongue; he bites at them, annoyed, and she jerks them away. Helen now fingers her own lips, moving them in imitation, but soundlessly.)
Martha: What you do, bite her hand?
Percy: That's how I do, she keep pokin' her fingers in my mouth, I just bite 'em off.
Poor Helen, she remains completely bewildered as to why people open and close their mouths intermittently. She desires to find out if and when people are doing this strange thing. Because she can't see or hear, the only way she can find out if people are moving their mouths is to feel them with her fingers.
I'm sure this particular exchange shows that she has recently realized that the tongue inside the mouth moves, too. Although we are not given any indication in the text, perhaps she has also realized that there is vibration coming from deep within the throat. One can understand Percy's annoyance.
No adult is around to chastise Percy, so why not get Helen's annoying fingers away? She can't tell anybody, so why not bite those probing appendages? Therefore, this exchange also shows how Helen is a constant annoyance, even to the servants.
Ironically, it's a fairly important exchange because it's how we understand Percy's annoyance again later when Annie tries to use Percy to help teach Helen.