Typically, it is allowable for external professionals to sit in on child contact sessions, in both supported and supervised contact. There are many reasons why this might be necessary, which can vary from gathering information for PAMS assessments to making observations for a section 7 report.
It is advisable to contact the local centre to see what arrangements can be put in place. The family you are working with, will have been told what information is being recorded and how this will be shared. It is therefore, important that you also make sure people are aware of the scope of your attendance and how you will be using any observations made.