It looks like international Swiss tickets go on sale much earlier. Do you know when?
It's not one month, but four in this case. Today the last day for which you can buy Switzerland-Italy discoutned ticket is September 7
I think I messed up.
Don't worry, everything can still be fixed. It's just not as straight forward with routes like Interlaken-Venice
So, in case you ok with fixed date and train, then:
1) go to
SBB ticket shop, under 'Travel to Europe' select not 'Tickets', but 'The lowest fare to direct connections to major cities' instead. It is place to find discounted tickets. And here starts the magic... since in your case (with departure 13:29) you anyway will need to change trains twice (in Spiez and Milan), it doesn't make sense to buy one through international ticket for almost 400 Swiss francs (that is full fare with a minor discount for children), but instead buy three separate tickets and save quite a lot. Thus in this part of SBB ticket shop you input Spiez-Milano Central (29/06, 14:05-16:38), this is what you should get
As you can see full fare on this segment is 55 CHF, while discounted tickets start from 23 CHF. Unfortunately for 14:05 those are already sold out, so only 36 CHF are left, but it's still a good catch (meaning 36*4 passengers=144 CHF)
2) Now we deal with Milan-Venice leg: cheapest option here will just not to bother with any ticket purchase, but already in Milan buy tickets till Venice for regional train departing 17:25 (arr.20:51). Three adult tickets (in Italy child fare <12 years) + one child will cost you only 62,25 euro (app. 75 CHF)
If you want a faster journey, then take 17:35-20:10 high-speed train, but full fare 3 adults + 1 child (as you can see on the picture above) cost 122,5 euro (that is full fare and what you pay at the station). Unfortunately for this 17:35 train all the discounted MINI offer tickets are already sold out (and almost no chance that will be available anymore), so with this departure you can't really travel any cheaper than 122,5 euro. The choice is yours, meaning whether arrival 40 min. later (20:50 instead of 20:10) worth 60 euro difference
3) Now the easiest part - Interlaken-Spiez. Since it's really short segment, just buy the tickets already in Interlaken, they will cost you only 31,2 CHF for all 4 (full adult fare is 10,4, while children get -50%) from Interlaken Ost or 29,4 CHF from Interlaken West (which is actually closer to town centre)
As result just by buying now tickets Spiez-Milan (while rest will be bought already in Switzerland and Italy) your whole family can travel Interlaken-Venice for as low as 250 CHF* instead of full fare 392 CHF (while keeping pretty much the same travel time and number of changes). Imho, not that bad to save almost 150 CHF in such situation =)
* or even cheaper in case you ok taking earlier train from Spiez (meaning to depart from Interlaken at 7:29 in order to catch 8:05 train to Milan for which 23 CHF tickets are still available)