It's not like children can buy overpriced phones or tablets. You can force the stores to help setup up restrictions for their devices for parents that don't know how or want to. Then start doing massive fines and restrictions on websites or app stores that host unregulated apps. If they can afford to buy them on their own, good for them. You don't use age to decide what's appropriate; I don't mind if my kids see nake…
Please don't do this to your children. Exposing children to pornography is abuse, regardless of the body shape of the subjects.
An teenage cousin showed me pornographic images when I was a boy, maybe 6 but I am not sure. I was confused by all the body hair the women had. I became physically aroused, something I didn't understand at all -- I assumed it meant that I suddenly needed to pee. The images were of "normal" women but the experience was abuse.
My wife had a similar experience when an older family member was openly watching a pornographic video. She doesn't remember what it looked like as much as the feelings of confusion about her own body's response, which in her innocence she also interpreted as needing to pee. Both males and females are sensitive to this kind of sexual stimulation from a young age.
In human development has well-known progressions for the development of digestive, musculoskeletal, sexual systems. We don't give weaning babies steak. We know not to make elementary schoolers do dead-lifts or run marathons. And we know better than to sexually stimulate children with pornography.