may I ask why you think traveling on thurs afternoon & spending Shabbos in kauai is preferred over sun afternoon and spending Shabbos in maui? your suggestion only gives me an extra half day or so of Kauai, unless that's exactly your point
my thinking was that 7 days in maui allows me to use a 7 day marriot cert (easy to buy), rather then using my own points for 4 individual nights in maui and 5 nights in kauai.
Hmm.. Could be 4 nights + 1 free is better than a 7 night. I guess that would also require shabbos in Maui.
Another factor could be if you care about minyan. Chabad Maui might have a shabbos AM minyan (posted start time is 1015 so might not be so helpful for a litvak). I don't beleive kauai ever has minyan. Either way, this discussion probably belongs in the "which island to visit thread".
With a 2 year old, my personal preference would just be to stay in Maui. Maybe do a day in HNL if you need to connect there anyway for a flight back to the mainland.