Congratulations on your upcoming wedding.

I second CostCo. We were at a wedding in Kauai (actually in Poipu) a few weeks ago and got a bunch of leis from CostCo.

You might want to consider splitting up your group of 50 for activities. First, it's difficult to plan anything that 50 people will be happy with, much less with the age ranges you described. Second, logistics are going to be a nightmare. If you must do something, a picnic on the beach together, as long as you bring along games or something to entertain the little ones, might work. The beach in front of the Hyatt (Shipwreck Beach) would be close if you're staying in Poipu. There's public parking and public bathrooms on the east side of the Hyatt. Poipu Beach Park is right downtown but gets pretty crowded.

How about a beach towel for the gift bag?

Update: Poipu Beach Park does have a bunch of covered picnic shelters with tables. One is quite large and would easily hold 50 people in case it rains. I know you can reserve the the shelters, I'm just not sure how.