Because online tickets are not changeable or refundable (unless that option is purchased), and are for a specific carriage number and seat number, there is no need to "validate" a bar-coded, prepaid online ticket.
It might be possible to do this itinerary with less-express trains, but because Thalys is such a bargain three-months or more in advance, it's not worth the trouble and changes. There are far fewer unreserved seats than your last trip to Europe. Pay attention to which station in Paris your train goes to. The Thalys ticket includes all three Brussels stations, but you must change from a very frequent local train if you don't start at Midi/Zuid Brussels. You can pay extra for "any Belgian Station" if you are starting from another city that day. (Local trains are unreserved in Belgium.)