Caroline if you tell us your itinerary you will probably get some good information. Be prepared for

We are a family of 5 travelling for 6 weeks through Italy and France using fast trains seaside inn where possible. We come from Australia. I've been looking at rail europe site to get idea of prices. Has anyone used this site? Is there another site cheaper but reliable that people have used or should I go through a travel agent?
seaside inn Here is the site for trains within Italy. You can get deep discount for purchasing in advance. The summer schedules are not loaded yet, so use dates within a month or two to check the possibilities if you are traveling after the first week of June.
Use bahn.de for scheduling information and buy through the country's' railroad system. Deep discounts are available if willing to purchase in advance and lock in your schedule. Generally the discount tickets cannot be exchanged or refunded so be sure of your schedule. Rail Europe is a travel agency and not a train company.
Caroline if you tell us your itinerary you will probably get some good information. Be prepared for some critical comments though if its unrealistic. There have been several threads recently from Australians who have unrealistic itineraries. Are you travelling over Christmas? The reference to six weeks makes me think you might be.