Macpac FTW. Because of the excellent and adjustable back system.
I carried our sturdy young lady and a load of crap (picnic etc) in it on walks at 10,000ft and whilst the pack was so heavy I could only just lift it onto my back on my own, I had absolutely no trouble carrying it once on. No shoulder or hip pain – top stuff.
Oh yeah but do remember to really wrap them up loads warmer than you will be yourself – you’re walking around, they are sitting still freezing their lil arms and legs off. Really bundle them up, esp if small.
Wouldn’t use it in town though, as it becomes hard to negotiate crowds whilst avoid turning round and smacking other people with it or knocking stuff off shelves etc. Front sling for that, or shoulders.