That's an intriguing train of thought, neesaki. Personally, I don't believe that PR was sexualizing JBR by putting her in the beauty pageant life. My thoughts are below, but I would also be interested to hear if there was anything else that got you thinking along those lines (using JBR as a sexual substitute, using JBR as a hook in JR, etc.).
I'm assuming that you are talking about the beauty pageants? On the outside looking in, it is easy to be appalled by mothers who put their daughters in those events because it focuses so heavily on personal appearance and physical beauty. In our culture, and especially for females, beautiful means sexy, so when little girls' appearances are altered to appear more beautiful for pageants, it inevitably means they are made up to look more mature and sexually attractive. This coupled with the fact that they are showcased for the judges viewing pleasure, which is enough to make most peoples' stomach turn (mine included).
All of that being the case, the women who participate in this don't see it as sexualizing little girls. Many of the mothers, PR included, were former beauty pageant contestants themselves. For them it is about the performance aspect for entertainment; they dress in costumes but also perform talents and other skills in the show. And JBR was involved with singing and dancing troupes in addition to her beauty pageant work. Many of these girls are more or less forced to do this by their mothers who are living vicariously through them to some degree, but some really like getting dressed up, doing a little show, and being applauded or winning prizes for it.
My opinion is that PR wanted JBR involved because it was something she was into, JBR was her only girl (so she wanted to share common interests with her), and JBR seemed to be into it (whether that was just to please her mom or genuine interest, we'll never know). All of this is totally natural and not suspicious, except that it created an unhealthy focus on JBR's physical appearance and public persona. Then again, PR was motivated by image and materialism in all aspects of her life, not just JBR. I am a PDI, and I think that this angle is part of the puzzle.