The Panda washer does not hold a lot of clothes. If you pack a lot in there the clothes won't get clean because there won't be any movement. It would be the equivalent of just soaking them in a bucket of water.
Ideally, the Panda can handle a few shirts at a time. It worked well for us because we were able to run about 3 loads twice a week. With the 4 of you you'll probably be doing multiple loads every day. I know how dirty 2 boys can get.
The upside is it's convenient to have it right there in the trailer any time you need it. You also won't have to deal with broken down washers/driers at campgrounds. The downside is it's capacity is very limited.
I got both the Panda washer and Laundry Alternative Spin Dryer from Amazon for under $250. I'm very happy with them as they gave us the option of doing laundry in a travel trailer.