I have a kelly kid carrier and it is awesome. My two year old daughter asks to go for rides in it even. Tonight we did two miles before we got rained. I recommend the Kelly's.
I've used the Badlands 2800 and it works perfect. Just open up the wings of the pack and strap them around the kids. I put a pillow in the bottom sometimes to let them sit on something and then their legs go through the bottom buckle straps from the wings. I hunt and shoot with them in the pack. They love it!