Congratulations on securing the difficult-to-come-by Cinderella Reservation!
Is it worth the cost for a picky eater? I took my fussy eater princess to the
Royal Table when she was 5. Mostly what she ate was a few bites of chicken nugget and her ice cream. Then again, that's all she ever eats. So, did she eat 20+ dollars worth of food? No. Did we have a great time? Yes.
As with all character meals (and most dining at WDW) you're paying more for the experience than the actual calories on the plate. My daughter loved chatting with princesses. The characters at the Royal Table really make a show of talking with the children and engaging them as much as they want. We were lucky enough that my daughter's favorite princess, Jasmine, was on hand the day we dined (Cinderella is always there but the other princesses rotate occasionally), so she felt extra-special. You have to decide how much your particular child would enjoy meeting the princess characters and go from there.
As for your son ... There are sometimes male characters appearing at the Royal Table. We've seen Peter Pan, Aladdin, and Prince Charming there on various visits but, again, the characters change frequently, so I can't guarantee he would see a male character. Boys are indulged at the Royal Table with the gift of a souvenir sword (plastic) that he might like. Again, you have to look at your own children and your own pocketbook to see if the experience is right for you.