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How old is she?
Kleptomania would not be diagnosed in a child of this age - what we would look at is any other symptoms, like, if she is cruel to animals, if she understands right from wrong in other situations, and if none of those are present - we'd work on teaching her about stealing in several different ways.
Allowing the negative consequences to play out is effective, imposing consequences of your own is effective, and demonstrating how it feels and why it is bad to steal is effective.
If those other symptoms are an issue, you'll want to have her evaluated and treated by a child Psychologist. Behavior like (all) that at 6 usually leads to conduct disorder later in childhood and through the teen years.
The next time she comes home with an item you suspect to be stolen, WITHOUT SAYING A WORD ABOUT IT FIRST, bring her back to that person's house, and ask the other child how she was able to come home with this item.
Allow the other child to answer first. If it is stolen, make her giveit back and apologize. Also have the other family (mom and child) tell your daughter how it made them feel to be stolen from, and then consequence her by cancelling the next event on her schedule.
You have to nip this in the bud - sometimes a grand show like this is all it takes, sometimes you have to do this several times.
I am leaving my office right now... but will be back on later. You can send me examples of what has happened in the past (or typical scenarios) and I can send some other ideas back to you...
Lastly - don't by into her "really believing" - ignore that and deal with what you need to do regardless of her reactions.