But let’s not confuse two things.
In some cases, we would like to obtain empirical data, but it’s tricky to design an experiment to obtain that data, for practical or ethical reasons. The details of parenting strategy - how and when to teach kids about bodily autonomy, whether it should apply to grandma’s sloppy kisses - would fall into that category:
…perhaps you’ve answered your own question there about why reliable studies in this area may be hard to find. And the outcomes for others (potential victims of the children in the study) would be even more difficult to ascertain, all you would have to go on would be actual criminal convictions. Teaching children about their own right to bodily autonomy obviously goes hand in hand with teaching them to respect the autonomy of others.
But at a more fundamental level, the question of whether kids should learn about bodily autonomy and consent at all does not derive from empirical data, it derives from ethics. Children should learn about it for the same reason that they should learn that torturing pets is wrong. Bodily autonomy and consent is a fundamental human right that we all should understand. Recent increasing emphasis is not just trendy parenting, it’s because of our increasing awareness that so many adults are ignorant of (or willfully ignore) this right.