I've had a ton of experience using different carriers for my boys so here's what advice I can share....
We got a backpack when my first son was born and to be honest I think we've only ever used it once. Generally speaking they are just too bulky and big to be convenient, and I don't really know of any that you would have enough movement wearing one to be able to shoot. The closest thing to a backback that I've heard rave reviews about would be the ergo carrier
http://www.ergobabycarriers.com/babycarriers/category/carrier/ You can use that one on your front or back and I'm pretty sure you could carry even a newborn on your back in one although I'm not entirely sure about that as I've never actually used one myself.
I generally use slings for my kids. For shooting I prefer using a mei tai carrier, you can use that one on the front or back as well and I've even carried a newborn on my back before. The nice thing about these is that the weight is distributed evenly without any bulk so it makes for easy shooting, or working, or whatever you want to do. I've carried my nearly 3 year old in one of these as well, they are definately made to be used for a large age range. I made my own but there are a ton of places you can buy them. If you want something more custom made (lots of shops actually let you pick your own fabric) check out etsy.com and just search for mei tai's it should come up with tons of sellers.
Another decent option would be a ring sling, although they take a while to figure out. I've shot with my 10 month old on my back with one of these but he was starting to get to be a bit heavy with that sling by then. Same thing for those, if you want custom/hand made check out esty.com or most specialty baby stores carry them.
The other sling I use often is just a basic pouch style. These are great for getting baby in and out quick but they are not comfortable for long periods of time. I use that one when shooting when I know I just have to carry one of my sons in between targets or something. You can find pouch slings almost anywhere, but they are also really simple to sew if you or or your wife sew. I have lot's of links with tutorials on making all three of those slings if you're interested.
Anyway there's what little advice I have to offer... we're due with our 3rd kid next month so I expect to have to carry him/her anytime we head out to shoot again.