I might start by addressing the underlying issue, which is probably this: Yvonne needs considerable reassurance that there's enough love and approval for her, that she won't be left out, that she matters, and that she is "good." I would be quite direct. I'd say, "Yvonne, you cut very well. Jimmy is going to have the first turn with the special scissors, but when it's your turn, I know you'll do a great job with the scissors." Or, "I like you, and I think you're a really special person, even if you are not first in line."
I would also have a chat with her parents. It is much better to do this in person than by phone. I would call and say that Yvonne is doing well, and that I think it would be a good idea to get together to share what we know about her. Here are some tips for making the most of this meeting:
1. Sit as partners, as equals, not as the authority figure and the recipients of information. Sit together at a round table, rather than having the parents sit across from you.
2. Start by smilingly discussing each of Yvonne's strengths.
3. Tell her parents that at school Yvonne always insists on being first, and that you're wondering if this happens at home, too. Yvonne's parents may look surprised and say no, this doesn't happen at home, or they may say yes, she often does this. If no, say, "That's so interesting. I wonder why she does this only at school." If yes, ask the parents if they have any thoughts about why she might so urgently need to be first all the time.
My guess would be that Yvonne has one or more siblings, and that being first is so important to her because, in her mind, being last would mean being least, and being in the middle would mean being nobody special. This may or may not be the case. In any event, Yvonne's parents might be able to shed some light on the situation.
The third thing I would do is use humor. I might say, "Guess what, Yvonne! Even if you have the sixth turn, I think you're terrific! Six is a very good number. Let's all count to six because Yvonne is going to have the sixth turn." Or I might say, "Want to hear something funny? The best number today for a hand-washing turn is seven. Who wants to be the seventh person to wash hands for snack? You do, Yvonne? OK. Thomas, you be number one, Sarah is number two . . ."
In my opinion, playfulness and humor are wonderfully effective and seriously neglected teaching tools. In addition to the other strategies mentioned above, why not give them a try?