Just a thought, but how will you make sure that the paper is not one that some kids have already done, or will each parent provide their own?
Yes-you would have to write the exams yourselves as you won't know which papers the children have done. A friend of mine did this a few years back as no mocks were organised around here. She said it was an absolute nightmare. She spent hours researching past papers to create her own VR one, had to choose an English text and write comprehension questions on it and the same for maths (it might be different in your area), had to pay to hire a small church hall, spent hours and a fortune on ink photocopying the papers, ages marking them and then a day writing marks/rankings for the parents. I think she was very out of pocket by the end!
Price it up before you rush into it-it might be cheaper to do a paid mock!!