Beneficiary's certificate certifying that no prisoner and no children have been emplyed in the production of products comprised in this L/C.

这并不是一个完整的句子，而只是一个名词词组，即Beneficiary's certificate 被现在分词短语 certifying that no prisoner and no children have been emplyed in the production of products comprised in this L/C 后置修饰。现在分词短语可以改为定语从句：

Beneficiary's certificate which certifies that no prisoner and no children have been emplyed in the production of products comprised in this L/C.

comprised in this L/C.为过去分词短语作定语，修饰products, 等于 included in this L/C.