Another Eurorail question

Bookings are generally available 90 days out, though it does vary with different railway companies. October is too far in the future.

Reservations are not necessary for Netherlands, Germany or Switzerland. Though if you are going from Amsterdam to Interlaken in one day they would be useful to ensure you can all sit together.
Once you cross the border into Italy reservations are compulsory for high speed trains, which is what your trains will be.

By "Eurorail" do you mean you are planning to buy passes?
Are you doing any travel in Netherlands or Germany other than the one trip Amsterdam to Interlaken? If the answer is no, it would be cheaper just to buy normal advance-purchase tickets (which include the reservation) for this leg; possibly a local pass for travelling around Switzerland, and normal advance-purchase tickets for Italy (which are a lot cheaper than a pass).

I partly agree. Although many trains in Germany, Benelux, and Switzerland don't require reservations, sometimes its still good to get them as certain times can book in advance (especially long distance international day trains).

As for rail passes, its not as clear cut anymore. Sometimes they are a good value and sometimes they aren't. You really have to do the math and add the cost of the individual tickets and compare them to the cost of the rail pass. For these three routes you had mentioned standard tickets will run somewhere around $630 pp for 2nd class tickets. For a 4 country Select pass saver for 5 days, you'd pay about $380 per person plus reservations if they are required. Reservations are an extra charge on top of the rail pass. As you can see a rail pass may actually save you some money.

For advanced purchase discounts, you can get those too but I think you'd be hard pressed to get three tickets that would be less than the cost of that pass, even with reservations. Additionally, the pass would be in 1st class and you'd have 2 extra travel days to use how you want. Either way, compare your options to see what is the best value. Whether you go with a pass or not will depend on the type of trip you are doing. I hope this helps some.

Thanks for your information. @Chris, we are not planning to do any other stops on our way from Amsterdam to Switzerland. I heard, if you did buy separate tickets, then one should buy train tickets from Amsterdam to Cologne, Germany and then buy your ticket to Interlaken, Switzerland so you won't be charge a tariff tax. I'm not too sure about this. I'm aware to buy tickets for short trips in Italy. If they had Euro Pass for 3 countries, that would work great for us as Italy tickets are fairly cheap.

Tariff Tax?? Sounds like an urban legend to me. Nonetheless there may be some good financial reasons for breaking up the trip a bit. I see on a random July date, Deutsche Bahn will sell you an nonrefundable ticket on an ICE direct train from Amsterdam Centraal to Basel Bad for 99 euro, but if you change to an EC train in Duesseldorf, you will get in about 50 minutes later than the ICE, but the cost is only 39 euro. Plus on that route you will get a tour along the Rhine River.

"one should buy train tickets from Amsterdam to Cologne, Germany and then buy your ticket to Interlaken, Switzerland so you won't be charge a tariff tax" - that one is news to me. I don't even know what a "tariff tax" is.

For 6th June (a random date), SBB ( www.sbb.ch ) will sell you a ticket from Amsterdam Centraal to Interlaken Ost at CHF 235.00
www.loco2.com is offering the same trip for £171 (roughly the same)
DB ( www.bahn.de ) will also sell you a ticket.

"We are planning to use Eurorail for 4 countries."

I hope you mean a Eurail pass for 4 countries, not "Eurorail" as if it is a rail line. The railroads in Netherlands, Germany, Switzerland, and Italy are each run by the national rail companies in those countries - Dutch Rail, German Rail, Swiss Rail, and Italian Rail. Eurail is not a railroad, it is a consortium of the national rail companies in Europe organized to package and market rail passes.