What pass to buy - Need your help

and my kids love the idea of going around in water taxi

There is a world of difference between a water taxi and a vaporetto, just as there is a difference between a taxi cab and a city bus. Water taxi is a speed boat the seats about 10 people and takes you where you want to go exclusively. Pretty much 100 EUR minimum fee. A vaporetto goes on a set schedule, makes many stops and holds maybe 100 people. Nominal cost is 7.50 EUR per person, per ride. A pass will give you unlimited rides.

There is something called a Rolling Venice Card, that gives a discount for young people. It comes only in a 3-day variety, so that erodes some of the cost benefits in your case. Check that out.
http://www.veneziaunica.it/en/content/rolling-venice

with only 2 days, you don't have much time to do all this. You really only have 1 full day, and an evening and a morning. Definitely skip the La Fenice tour, you don't have time to waste.

Where are you staying?

Sorry I meant Vaparetto instead of water taxi. Water taxi is good but the number of rides my kids may want, I may end up bankrupt.

I'm staying at Hotel Carlton on grand canal.

I looked at the option you provided. Regarding Rolling Venice - It says following

PLEASE NOTE: You can purchase ROLLING VENICE on VENEZIA UNICA ONLINE only with ACTV Transport Ticket (3 days / 3 days + Airport transfer one way / 3 days + Airport transfer return ticket)

This doesn't apply to me.

Any other option I can try ?

Let me know.
Thanks

Looks like I will have to buy first a Rolling Venice Card - that is 6 Euros and then € 34,00 instead of € 52,00 : ACTV “3 days Youth pass” + Transfer MARCO POLO Airport (RETURN TICKET)

That's total of 40 euros so I will save 2 Euros per child ? Is my math correct ?

If I skip La Fenice tour - would option 1 (I mention above) give me an option to atleast go and have a look inside La Fenice or would I have to buy a seperate ticket for it.

Can I buy all passes online. I'm assuming my 2 days will start the first time I use the pass. Is that correct - or does the time starts ticking the moment I buy it.

Thanks

The 48 hour vaporetto pass with return airport bus goes from the time of first use. Should work out for you if you land at 2:40 pm, it will be at least 3:10 pm before you have your luggage and buy the passes are out front and board the No 5 bus where you scan your pass as you get on the bus and starts the 48 hour clock. You have a 4 pm flight 2 days later and you should be at the airport at least 1 1/2 hours before your flight. so you should be on the 2:10 pm bus at the latest, arriving at the airport at 2:33 pm. That is less than 48 hours total, so your pass will be valid the entire time.

The Hotel Carlton is walking distance from the Piazzale Roma bus depot in Venice. About 300 yards.

If La Fenice opera house is of special interest to you, then go ahead and add if you want. Otherwise skip it. Nothing going on there as far as performances while you are there.