Anyone using the Easy child system | ADHD Information

I use the marble system in a way.  I use it for both my boys (10 and 7 and 7 has ADHD).  I do not take it away for negative behavior but grant for positive behavior.  I found something he really wanted (car magazine or lego star wars things) and he works toward those by earning marbles for chores, behaving, doing homework w/o complaining, gettingdressed and ready in the morning and it has worked wonders.  I know this was mentioned in the Marble thread but I was wondering how many people have used this computer based behavior system?  I have to do something so Im looking for ideas.  I do have a question and Im sure you veterans can help me out.  Whether I am using this or the Marble system, what do you do when the child disputes getting/taking away marbles?  Such as if you have a -5 value for arguing with parents and you say take away 5 marbles what do you do if they say I didnt argue?  What if mysterious marbles(checks) appear on the chart and the kids says, but I did put away the toothpaste it was (insert sisters name here)! How do you get away from the bargaining and arguing about yes I did that?  HELP!!!  also, another question if you guys dont mind- Do you use this for all your children or only the problem kid?  My other girls are 15 and 13 and are really no trouble at all. Do I make up a list for them too eventhough it is really just my darling 10yr old that needs work?