My daughter and I plan to ride the Transamerica Route in 2014 using Adventure Cycling maps. I'm very map oriented (experienced with reading maps, etc.). My daughter is good at reading maps and navigating too. Each of us will have an iphone, but we'll probably keep them in airplane mode to save battery unless/until we have a reason to use them (e.g. google maps if we have a question about where we are, etc.)
Thanks in advance for your input.
A young girl with an iPhone? Your main concern will be powering her phone. If you have the phones, invest in (more than one) external power supply and use and enjoy their advanced features. Get an interface for the bike if you like.
But even Google maps can be inaccurate.
Remember that the first cross-continental trips were made without phones, without maps, without GPS, without company, without sponsors. You can survive. Heck, leave the phones at home and see what living in the 20th Century is like.