* Children at risk from collapses, mercury poisoning
* Government says committed to ending the practice
* Tanzania is Africa's fourth-largest gold producer
By Fumbuka Ng'wanakilala
DAR ES SALAAM, Aug 28 (Reuters) - The Tanzanian government said it was committed to stamping out child labour in small-scale gold mines after a U.S. rights group said thousands of minors were at risk from ...