You will find tons if info on gear and advice for tiring with kids in the forums. I started with my oldest son at the same age as your daughter. Initially with a TAB (Burley is much better than Adams) then went to a tandem and now that my second son is 6 we ride a tandem with TAB behind it. Six is a perfect age to start and plan 40 miles max loaded. We started with organized supported rides like the MS150 but they have now raised the min age to twelve so you would have to look at other supported multiple day events. The other fantastic routes we have ridden are the C and O and Great Allegheny Passage rail trail route from DC to Pittsburgh and the Erie Canal across NY State. Both routes are almost entirely off road so you have no cars to worry about and bike traffic once you get out of DC is only moderate. There is also a great week long on road route around Lake Champlain in NY that we are planning to do next year but we have just started to plan that so cannot speak about it personally yet.

I am lucky that I have two brothers in law that have kids the same ages and I got them interested in tandem riding too so we have great family trips with just the dads and all of the kids so it is really fun for all. We have had some really memorable trips so far and it has made all of the cousins a lot closer.

Let us know what ever we can do to help you get started. This is the best thing I ever have done with my kids and now they do not have such a car centric mindset to everything. When else will you get a chance to spend eight hours a day within two feet of your kid just talking and no TV or video games?