What criterion must otherwise unapprovable research involving children meet to be considered by an IRB?

To be considered by an IRB, research involving children must meet the criterion of presenting an opportunity to understand significant problems affecting children's welfare. This is because the primary concern of IRBs when reviewing research involving children is the protection of their welfare and rights. This option is correct because it ensures that the research will be of benefit to children and will contribute to their well-being. Options A and C are not sufficient criteria on their own, as direct benefits to participants may not always outweigh any potential risks, and parental consent does not guarantee the ethical conduct of the research. Option D does not specify the need for approval from all guardians, which could potentially compromise the rights and welfare of the child. Therefore, option B is the most appropriate criterion for otherwise unapprovable research involving children to be considered by an IRB.