since the description of the seating you provided is in Italian it looks like you may have used the Trenitalia website or a third party to reach that page.
I'd advise to use the SBB website which is the official Swiss railways website, and click the EN for English.
If you you buy from the SBB the seat reservation on that train is included. While it is in Switzerland (the bulk of the ride) it, like all Swiss trains, is unreserved seating. Once it crosses into Italy everybody on the train has to have a seat reservation. So you will have seat reservations for your whole journey - that will stop people popping into them in Switzerland.
Like Lola I was going to recommend a read of the whole page at https://www.seat61.com/trains-and-routes/switzerland-to-italy-by-train.htm
Your EC 173 has the logo TT, and is a tilting train as described on that page.
It is a 3 and a half hour journey plus or minus and it is perfectly acceptable to bring (hopefully not too smelly) food and drink on board with you and picnic on the way. Snackies for the kiddoes too. There is a Migros food counter and small supermarket in the Luzern station as well as lots of other choices.
Unlike some Swiss double decker inter-regional trains these trains do not have a FA Family Car with playground, slide and dinosaurs.