Giving mineral oil laxatives are comparitively safe for children. But it also depends on the age of the child as well as the frequency of childs problem. Using this laxative once in 3 months might not be hazardous if used in small portions.
Yes, they are perfectly safe for your children. They are however maybe not the best choice. All the mineral oil will do is lubricate your child's stool so that he/she can use the washroom. You may want to look into a stool softener as well, as perhaps an excess of fiber is the cause.