we had 3 younger kids at our wedding (all under age 10). They loved it there! actually they spent the entire day there with me and kayaked, hiked, snorkeled...all that fun stuff.
Yes it is fine there for kids BUT you have to remember that there are no actual lights there once it gets dark. everything is lit by torches or candles. so yes a child (or anyone really) could easily trip and fall once it gets dark, that in my opinion would be the only "safety hazard", especially making the walk back to the boat from the reception area. But really it is fine for kids.
Nobody got sick on the boat out of my guests either but they all did say it was a VERY VERY choppy ride. I think that though is because the boat was late leaving PV with my guests (my guests fault not Vallarta Adventures) so they had to book it out there. all the boat rides though that I have been on to LC and back were very smooth, including for the wedding I have been out there 3 times.
oh! and what was your question about December weather? I did my site visit in December...the weather was beautiful! a bit chilly in the evening but not cold by any means. We did not go though during the holiday season, more the beginning of December so yes I can imagine that flights will be more expensive for your guests in late december just because of the time of the year. If you give them enough time to plan/save you should be fine.