I find with my six year old on the back gears are my friend. On the flat I can pull off 48x12 no problem, but get a good uphill and gets very hard very fast. I'd at least go for 1x7-10 group on the back to give you some options. I like 3x7 because I can granny gear up anything with her on the back with that setup. The only problem is the extra weight on the back makes it very light in the front and if you get too steep its not very safe.