From someone who had had 13 holes artificially installed in his face, I've got the same rules. Ears don't get pierce until the kids are roughly physically mature. A growth spurt doesn't happen evenly and can cause the ear holes to be very off-center.
1. Ears only. The rest can wait until they are adults.
2. It has to be the kid's decision. No 'peer pressure'. If the kid can't convince me of that, he(or she) isn't ready to get pierced.
3. They've all got to wait until about puberty. Young growth spurts are too unpredictable.
4. No tats. Kids aren't mature enough to make reliable, life-long decisions.