A little more research on the net and I discovered the following.
The tragic event happened on Friday 13th 1955.
Mine clearance took place in 1945, repeated in 1947 and again in 1949, eventually a clearance certificate was issued on the 17th February 1950.
Of the 117 mines laid in 1940, 5 were lifted in clearance there was evidence of the existence of 54, the remaining 58 are still unaccounted for.
The five young lives lost on the beach, one body was never located despite a major search by the Coastguard from St. Aldhelms right round to Poole Harbour and all the way to the Isle of Wight.
In the report made by the Officer who did the clearance, and from the inquest in 1955 it was stated the bomb that killed the children had probably been swept inshore by a gale.
Now I have the date of the incident I shall be researching our station logs to see what else I can find. If any readers have any information on the beach defences or the incident itself, please do get in touch at firstname.lastname@example.org