If you have any sewing skills, you could make them a simple vest. I still have my GS Brownie vest from back in the day and would just use that as a pattern. If you made it a little big, they could use it to play with for a couple of years.
Are they old enough to use a camera? You could possibly give them each a disposable camera to record what they find on their safari trip that day. There are lots of interesting things to look at that are kid height at AK: foot prints in the cement, the animals, colorful artwork and patterns around the park.
And of course all good explorers need a hat for sun protection.
Just make sure that whatever you choose is quickly packable when you arrive at a ride and won't fall off easily.