Susan,
There are not as many options in 16" tires as some of the other sizes but there are enough to allow you a little choice. Schwalbe offers several different tires in that size. My son has a set of Schwalbe Marathon tires on his trike that have been going strong for over 4 years now. They have been flat free and very durable, but also zippy and responsive. They also make their Marathon Racer in this size. I have a set of those with over 4000 miles on them and they are still going.
 
Some other good brands to consider would be Primo Comets, Kenda Kwest, and Maxxis Hookworms.
 
Here is a link to a tire page that will show you these as well as some other tires available in 16". Make sure you know the correct size (305) or (349) before you buy. Both can be considered 16, but you will need to make sure you buy the same size tire you currently have on your trike.:
 
http://www.everybicycletire.com
 
As for changing wheel size, there are different schools of thought on that. Some people love to change things up and experiment with different size wheels. Others will tell you that changing wheel size can badly alter the geometry of a trike frame, leading to a trike that can be less stable. Probably the best reason I could think of to change your front wheels to, say, 20" (406) wheels would be tire availability. I can't tell you what might work best for you, but on a practical note, changing the wheels will be a lot more expensive than just buying tires.
 My son loves the 16" front wheels on his Pocket, but we did have to change his rear wheel to a 20" because he didn't have enough ground clearance.