W.M.
I have a 5 1/2 yr old and at this age, she is old enough to understand and to do what you are saying. I would try this approach. I would go and get some Kandoos, or children's wipes for the toilet. They are flushable wipes for kids. I would show her that you bought them for her and tell her one time that she is to wipe with these each time she goes #1 and if she does not, the next step is that you will buy her diapers so that she can be the baby she is acting like. I would tell her very serioulsy and be to the point and let her know that this is the end of her not wiping. You explain to her what kind of infection she could get, how she might smell like pee to her friends, etc If she does not do this, you will HAVE to follow up with what you said and buy or borrow a diaper or two from a friend to scare her. I would even borrow one first and set it by the toilet next to the wipes. I would tell her that she has the choice, you are allowing her to make the choice between being a big girl or wearing the diaper. Mommy is no longer going to wipe you when you go #1, I will continue to wipe you when you go #1 until you are ??? age but this is it. "Do you understand mommy?" "Do you have any questions?" and go from there. Stick to what you say and it should work. My son always listens when I put it to him in those terms.
Good luck, W.