I have no personal experience but I'm sure MaillotPois will be able to help!
I have a (male) friend who's been riding a "road train" for a while: he's in the front, his now perhaps 10-yo daughter is in the back of the tandem, and his now 6 or 7 yo is on a third-wheel contraption at the back. It's pretty impressive to see him climb up the local hills. You can get an idea of the experience here: http://mgagnon.net/velo/index.en.html
He describes only two "short" (150 km based on my cursory glances) tours he's done with the girls, but I know he's been out with them on trips for literally thousands of kilometers. I remember hearing that having an interesting destination (like ice cream) was important. I guess building up distance is also important. You can't go 100 km the first day, especially not with the youngest.
I'm sure cool bike clothes "just like mommy" will help, if not physically at least mentally.
Keep us posted! (And show us some pics!)