July 21, 2012? Maybe you meant 2013?
Anyway, since there is no direct train from Paris to Vlissingen you'll to make change and here you have two options: a) make one change in Rotterdam (high-speed train
Thalys Paris-Rotterdam and local dutch trains from Rotterdam to Vlissinge) or b) make two changes (high-speed train Paris-Brussels, then train from Brussels to Roosendal and finally Roosendal-Vlissingen). Both options roughly take around four and a half hours and will cost pretty much the same in total
Regarding most economical option it makes sense to consider purchasing in advance discounted tickets from Paris to either Rotterdam (or, if you go with option b - Brussels), that way you'll have a promo fare tickets for the most expensive part of the route. Tickets can be bought via official tgv-europe website*, sale is open for up to 92 days in advance, so if you check now dates in January or early February you can see how cheap the tickets can be (Paris-Brussles start from 25 euro, while Paris-Rotterdam from 29 euro), for July you can count on more or less similar discounted fares.
Next, when you already in Brussels or Rotterdam second ticket till Vlissingen, full fare standard ticket price for Rotterdam-Vlissingen bought in Netherlands is € 18.60, full fare international ticket Brussels-Roosendal-Vlissingen is € 29,60
In regards to Vlissingen-Amsterdam (or any other rail journey within Netherlands) we can just remind you about special offer from Dutch Railways for passengers with children -
Railrunner. On the same ns.nl website you'll find also all the other necessary info (timetables, fares etc.) when traveling in Netherlands
* tgv-europe website is a bit tricky, so in the beginning when on website they will ask you this:
Make sure you click on "Continue on tgv-europe.com", that way you won't be redirected to North American reseller website. Or, as an alternatively, from the very start you can simply click "Other Countries" at the bottom left without clicking on "America"