If underage children are accessing explicit content, it is the parents, not the websites that are at fault. Why are you handing children devices that are not parental locked to only permit applications that have been approved by the parent?
If the parent fails to properly vet the apps they allow their children to use then it is the parent that needs to be questioned. Pushing this requirement onto websites and services is moving the goal posts. If the parents are too technically inept or unwilling to police their children's devices then they should not be handing them to their children.