Hi, Marie. I used the MSSB about 15 years ago -- so I'm not the most up to date on the literature about the measure. But, I found it to be an excellent way to tap into the inner representational world of school-aged children. However, we didn't find robust differences between the negative aspects of the representations of self/parent in our healthy children compared to an emotionally disturbed sample. I think this was in part because traumatized children may (at least in some cases) be less apt to open up with an adult examiner. So, that was my only problem with the measure. But, you will get a lot of fascinating data. I was going to suggest that to assess attachment you might want to consider using the child attachment interview (CAI), but it's only been validated for children down to age 8, and I think because it's an interview, the verbal abilities of 6 year olds might not be adequate to get good data using it. I hope my response is somewhat helpful. Good luck with your work.
Thanks a lot for your accurate response. I also think about the Attachment Story Completion Task (ASCT; Bretherton et al., 1990). I already used it, but it's quite long for my actual research.