I am an extremely big fan of horror and very much enjoyed your story. The dialogue is done very well and is very believable, inside of her head and with the painter. You did a wonderful job of distinguishing the voice in her head, from her own voice in her head and the painter's voice. It was a sequence that could have been very confusing and you did a wonderful job. You left it open, what she was doing it for, why a little ten year old was to kill thirteen and if the "Voice" was actually gonna let her go after reaching her number. That worked for me, not all stories have to have all the loose ends tied up.

Your writing style is very strong and unfortunately that's where I have a negative comment- 

 The painter screamed and began to flop around the floor.

Everything before and after was so strong that this sentence just seems extremely flat, weak compared to the rest of the story. I also appreciate the use of the color red, though instead of pointing out her eyes are red and the paint on the floor is red, you might want to use a different shade of red-crimson, auburn...

I did enjoy your story, thank you.

     I really enjoyed the mood of this work.   From the very opening of the story,  I could really feel the sense of unease.  Also I feel that the nature of the voice was vague enough to be real or the product of insanity, giving the story another layer of depth.  I only have a few miner points that i feel could be improved:

‘Jenevivve,’ she heard in her head.

The name is usually spelled "Genevieve" .  Since it is a Proper name it really could be spelled any way, but the unusual spelling you used is jarring to the reader.

She did as she was told, and tears started rolling down her cheeks. Why am I doing this? I’m only ten. How can I even think about doing this? Why…? She paused even in mid-thought. Why do I want to kill him?

 

until this point the girls age is in question, I thought she was in her mid to late teens at least, perhaps her age should be established earlier in order to make her plunge into insanity more dramatic, perhaps even a section describing her looks or the painting that the artist is creating, so the reader would better be able to visualize the murderous little girl.

The painter screamed and began to flop around the floor.

The imagery here was weak.  After the vividness of the earlier sections, this was a letdown.  The death should be fleshed out a bit more, perhaps describing the patterns the blood makes on the still drying painting?

In general I really liked it, it has a lot of potential and the characters seemed real to me.

This is a fine little slice o' horror you have here, and the best part is, you don't explain everything. The twisted possibilities are what make it so much fun. Now I hack it to pieces 

Adverbs are rarely necessary, and they tend to sound childish. "Foxily" and "Sinisterly" jumped out at me as the oddest. Try always to take out the adverbs and then read your sentences without them to see if it's absolutely necessary to what you're trying to say. If it is, try to write it a different way.

lest the painting be ruined.

Watch out for archaic stuff like this. If it fits the whole of your writing, you might be all right, but usually it sounds pretentious or out of place. Also, I don't think it's so imperative that subjects of portraits never move a muscle, but that's not my field, so I'll shut up about it. But maybe look into it.

“Miss,” the painter said, “would you please not move?”

The painter keeps calling Jenny "Miss". She's ten years old, and wouldn't he know her name anyway? It's very formal. Also, when I first read this line, I thought he was being a bit harsh and cold; now that I know the girl's age, he's just a bastard.

began taking off his apron.

These are bad habits of every writer; you just have to train your eye to notice them. Look out for passive sentences, and try to make your phrases more active. This line could have read "took off his apron", and it would have been more active and better to read.

a vane attempt

"a vain attempt"

the painter’s irregular steps cross the room.

"irregular" puts an image of the hunchback of Notre Dame in mind.

No less, no more. Well, not unless you want to.

This dude needs to make up his mind. I was stuck on this for a second, wondering "not unless you want to what? ...oh, the 'more' part." He could say "No less, but more is fine" or just stick with "No less, no more", but the iffiness makes it seem like there's no real point.

I’m only ten. How can I even think about doing this?

This is very unnatural. Would she really think that?

like one who had used such an object before.

We already know she's used it before; this seems redundant.

the mess on the floor that had been the painter.

My favorite line in the whole story. Very twisted.

Sorry, I feel I've hacked your beautiful story into bloody bits. They're just my opinions, and I say them as I think them. Heed them or toss them, you're the writer    It was a fun read!