Don't Give up! For some great articles on bicycling with kids, visit our magazine archives at http://www.adventurecycling.org/library/index.cfm
. Type in "Paula Holmes" in the search field. Paula has written many articles for us on kids and bicycling. Her latest article "Finding Manemones", is a great testament to a key in bicycling with kids - including things that interest THEM in your trip.
It is also very important to include the kids in the planning of your trip, starting with small rides and working up to something longer if you wish. If one of your kids is a big basketball fan, bike to a game. If another loves ice cream, bike to get a cone. Ask them what gets them excited, and plan a trip around that theme (taste testing all of the local cookie joints is a personal favorite). Make sure to discover what sort of bicycling, if that had to do one, they would choose. For example, do any of them like mountain biking or trail riding? If so, try to encourage them to get more into whatever sort of cycling strikes their fancy.
Since your kids are such varied ages, it might be nice to invite some of them to bring along a friend on a ride. You can also set up some system where you track their miles, and offer rewards for when goals are reached (like they pick a movie to rent when they bike to school five days in a row).
I would also recommend touching base with any local or state bicycle groups in your area that work hands-on with kids on bikes, they may have some great ideas on ways to kids even more psyched on bikes. The Thunderhead Alliance has partners in almost every state that work with bikes and kids. Check out who is in your state at http://www.thunderheadalliance.org/links.htm
And good luck!