A second sarcophagus has already been opened and removed from the site. It contained the headless remains of a child.
The Press Association's coverage was a bit more clear:
The head of the child appeared to have been removed and placed elsewhere in the coffin, which was an unusual but not unknown practice in Roman times. It is possible the burial included the remains of an older person in the same coffin.
We've dealt with headless burials before in these pages ... here ... here ... here ... etc... they are somewhat mysterious although I personally lean toward 'execution victims' as an explanation rather than 'ritual'. A child (some reports suggest he's about six years old) is a bit of a spanner in the works, though.