Ignoring the protective barrier around the exhibit, the boy was placed inside the open casket at Prittlewell Priory Museum in Southend, Essex. It tumbled over and a chunk fell off.
The museum's CCTV captured the culprits running away after the accident without reporting it on 4th August.
Conservator Claire Reed told the Southend Echo that staff were "shocked and upset" at the "unbelievable incident".
She added: "Staff heard a thump and that was the first indication something had happened.
"It was one of those isolated, terrible incidents. It's a very important artefact and historically unique to us as we don't have much archaeology from the priory."
The coffin was found in the grounds of the priory in 1921 complete with a skeleton which could have been a senior monk.
Ann Holland, executive councillor for culture said Reed is assessing the damage to the coffin and will carry out the repair using materials and techniques suited to the object.
She said: "To prevent future damage we now feel that the coffin needs to be completely enclosed and the curatorial team are assessing how this can best be done.
"The area will reopen as soon possible. In the meantime we would like to remind all visitors that they should observe and respect any barriers and signs in place that are there to protect our important heritage and history."
Although already in three pieces, the sandstone casket has been on display since the 1920s.
