This would be a second interview or third depending on how many questions were already asked by LE. IMO, by the first or second interview, LE should already have a good idea and possible theory as to what may have happened to the child. There would be no need to keep reinterviewing the parents a third time.
Interviews:
1) The initial questions following the 911 call. The basics. Timeline, child's clothing, photos of the child, etc.
2) Possible persons of interest, review timeline, employers (if any), names of family members, friends, coworkers, neighbours. Questions about the child. daycare, health issues, siblings, etc.
Then it's up to LE to investigate everything. If stuff was found in their investigation that needed to be addressed then bring in the parents again for an interview.
In this case, they did it backwards, imo. Show the parents (mom) the burnt clothing which may or may not have been the child's. Accuse her of killing the child.
Of course, I'd be frantic if I was innocent. This clearly shows they have tunnel vision. I would do exactly what mom and dad did in this case. Lawyer up