Honestly I think they just liked the 'what if' scenario of adults suddenly becoming kids. And the premise they chose forced them them to deal with somebody the person is in a relationship with, which they couldn't really get any closer to than having an uncomfortable hug on network TV.
What I don't get is the way the crew was unable to take commands from child Picard. I liked the way Stargate handled the same premise better. At the beginning everybody was raising their eyebrows at taking orders from a child, but then when he started talking like O'Neall, all the men could immediately tell it was him and respected his authority. Picard should have been able to demonstrate his authoritative Picard-ness and immediately win over the raised eyebrows.
Of course Stargate ended on kind of a creepy note, where child O'Neall was actually a clone of O'Neall and ended up going back to high school with the clear intention of hitting on high school age girls, which I would argue is more wrong than any affection between Keiko and Miles.
Although I would argue the only thing immoral about the topic is the lack of ability of the person to consent.