While having sex under God's plan can have many benefits (even beyond procreation which is the ultimate reason for it), sex with kids has no benefit that I can see.

Please list some of those "many benefits" and explain why they do not apply to relations between adults and children.

The only benefit I can think of that doesn't apply is procreation. Besides that: emotional bonding, pleasure, stress relief, increased self esteem, and all the others seem to apply to all loving sexual relationships regardless of the age or sex of the partners.

If anything, it would seem when you bring age into the picture it presents new benefits related to mentoring/teaching about sexuality. The rise of cougar culture in the US has brought that aspect to the forefront lately.