If by that you mean passes that do not include long-distance travel, they probably are not worth it.

I am thinking about taking my parents (seniors) and my 3 kids (14, 12 and 11) on a Frankfurt to Paris to London trip next April. My hope is that we would be able to travel by high speed rail between the 3 cities but, if possible, I would also like to secure local rail passes just in case we would like to do day trips to nearby destinations while we are in Paris and London. We have tentatively planned 7 days for this adventure. Any suggestions as to the best and most economical ways for us to get this done? Are there rail pass and hotel packages? What about hostels? This is my first time planning this trip, so any help would be most appreciated. Thank you.
If by that you mean passes that do not include long-distance travel, they probably are not worth it. Besides, in 7 days for two major European capitals does not allow for side trips (except maybe Versailles).
car rentals in denver colorado Three cities in 7 days is a lot and you want to take day trips. That means that you will have almost no time to see the cities. You need to allow time to get from one location to another. You'll car rentals in denver colorado need at least a half day for each segment. car rentals in denver colorado In addition to the train travel time you need to factor in traveling to and from the train stations.
Thanks for the information. That is a good start. I will definitely check sncf.com when ready, but what about all these different passes car rentals in denver colorado like the German Rail Pass, France car rentals in denver colorado Day Tripper and Britail Pass that I have found via Rail Europe? Also, do you know whether or not there are such things as rail and hotel packages? Or, are we better off booking rail and hotel separately?
@ Michael and Adrienne: thanks car rentals in denver colorado for the advice. I kind of thought that day trips would be too much for our 7 days, but can't blame me for thinking big, right? So, yes, we will, instead, wisely spend our time exploring key destinations in each city instead. Lodging. Any suggestions on lodging?
How much do you want to pay? You mentioned hostels which are about E25 or E30 per person. You can look at hostel world or other hostel sites and get an idea of prices. There are 2 threads here on Paris hotels car rentals in denver colorado - Paris right bank hotels and Paris left bank hotels. And there are lots of recommendations for Frankfurt hotels. Please use the search box to find the threads.
If you are coming into Frankfurt you could easily spend seven days just visiting Frankfurt and the towns and cities around it, even into Luxembourg. car rentals in denver colorado I would rather do that than try to include Paris and /or London. Both of those two are major destinations and just 2 -3 days is short-changing them. Or fly into Paris or London and just do them. Or budget a lot more time. 14 days would be getting closer to the mark, three weeks even better.
You could easily do Frankfurt and surrounds by rail as there is an excellent rail network for both local and major trains here. One of the Fodorites, texasbookworm, car rentals in denver colorado did just this trip recently - search for her posts and TR. There are also local rail passes you can use to make travel within Hessen or other states cheaper car rentals in denver colorado for groups of people, although I'm not an expert on those (although there are lots of people here who could advise you on that here very competently).
Realistically you can see quite a bit of either London or Paris, with 1 or possibly 2 day trips. Trekking through 3 cities would essentially give you 1 to 1.5 days in each city - barely time to see 2 or 3 sights and then move on.
If you have a full 2 weeks you can do 3 places superficially. But you need to understand that moving so many people, with obviously different interests, habits, eating patterns, etc is going to be a whole lot more complicated than travel just with a couple of people.
As for hostels - if you want private rooms - versus bed in common dorm rooms they are not necessarily less expensive than modest hotels - and many don't allow children. If you are contemplating hostels you should check out student resources (Let's Go Guides car rentals in denver colorado or Thorn Tree section of Lonely PLanet) to get info on them. Some are fine - but some are party central with lots of booze and even drugs.
As for accommodations, most hostels cater to the younger backpacker set and your senior parents are not likely to be comfortable there. You can try a site like www.venere.com and search for 2 star hotels.
If you do cut your itinerary to one city, another option might be to get an apartment. I'd suggest you look for a place with two bathrooms since you have six people in your group. You could save some money on meals, at least having breakfast at home. If you were to choose Paris, for example, on www.homeaway.com properties 419884 or 934046 look like they would work (just examples, not endorsing those specific properties)
While there's a general consensus on this thread car rentals in denver colorado of not rushing to see 3 cities in 7 days, and I'd tend to agree. (If it were me I'd skip Frankfurt and split the week evenly between London and Paris).
Day 2 - Check out early and dump bags in lockers in Hauptbanhof. Do a walking tour in the late morning and then early afternoon - train to Paris (about 4 hours). Stay in hotel in Paris (suggest either near Rue Cler, St Germain, or even better in the Marais). car rentals in denver colorado Dinner near hotel and then early night.
Day 3 - Do the free walking tour from Sandemans first day morning. You won't be entering the Lourve car rentals in denver colorado so take pics by pyramid. Tour ends at Place de la concorde. Spend afternoon browsing nearby Place de la Madeline. Have light salad lunch there - possibly the Fauchon bistro, then cab up the Champs Elysee, dropping off 3-4 blocks before the Arc de Triomphe. Take metro from there to Tour Eiffel - then hang round there at night till 9ish for the lights.
Day 4 - Morning walk the left bank, enter notre dame and seine. Skip ste chappelle (not worth it given 1/3 is covered in scaffolding anyway). Afternoon - tour palais garnier and quickly car rentals in denver colorado pop into the galeries lafayette food hall. Do a Montmatre walking tour in the evening.
justinwong's itinerary is (barely) doable. However you specifically mentioned the 'seniors' which would indicate you are concerned either about their health or mobility. Plus you have 3 kids. Are all 6 of you totally able to handle your own luggage schlepping on and off multiple trains and manage a very fast pace w/ very little sleep?