I've never done the luau. Personally, I wouldn't risk spending that kind of dough-re-mi on something where I'd have to bring a child that young. Sure, your kid is good...usually. But in the Orlando heat all day, off his regular routine and filled with Disney sensory overload, who knows how he'd react. Plus, to my recollection, I'm not sure how much of that food is appropriate for kids to eat. Hell, I'm not even sure how much "regular food" a 17 month old can eat, but I sure as hell wouldn't think ribs are good for a kid his age, even the mild-spiciness of the Disney ribs. The chicken is very fatty (part of what makes it so tasty), the salad is drenched in a tart dressing...I would suspect you'd have to try to have alternate foods ready for him.
Then there is the show itself. It's all good fun, but sometimes, people are loud, YEEEEE-HAAAAAA!!! kind of loud. I can't remember if there are pistols fired or not. I think not, but I can't honestly remember. But the singing can get loud, and the audience can sure get loud. Plus, add the factor that some people will be taking advantage of the all you can drink beer & sangria, and you might have some people being kind of scary-drunk for a kid to be around...and of course, Heaven forbid your kid just gets fussy, or wants to roam around while the performers are doing their thing (which often involves audience intermingling, imagine a kid getting caught underfoot during a show by a distracted performer)
I don't know much about the luau. I'm just assuming that it'd be more peaceful, but not necessarily sedate. Maybe sometimes, the music gets loud (and rhythmic), or the dancing becomes fevered...
Like I said before, I just think you really, REALLY risk not getting your money's worth & not enjoying yourself, bringing a kid that young to a dinner show. Depending on the crowd and the performers & how well-behaved your child remains, he could even be a distraction. I'm not down on dinner shows, and I'm sure you'll hear from other forum members who brought wee tykes to dinner shows and had a blast. I just think it's too much of a risk. If you're going to do anything (and if they're happening when you're there), I'd say do the Mickey's BBQ @ Fort Wilderness. It's a little cheaper and geared more for kids anyway.