If you are going to change the rear cranks, you might want to go for 165 for your daughter.
As for gearing, whether the 28 tooth cog gives a low enough gear depends on the size of your little chainring. Personally I would recommend you have at least a 22 inch low.
We rode the Southern Tier on our tandem (self-contained) and with a low gear of 20 inches (a 20 tooth granny and a 27 tooth big cassette). It was adequate, but when we headed to Colorado we switched to a 17 inch low gear (20 front / 32 rear). I think we will stay there. Our setup is 48/38/20 chainrings and 12-32 cassette.