ATVO or ACTV bus from airport to Piazzale Roma?

The No 5 ACTV bus only takes 21 minutes to go from the airport to Piazzale Roma and the buses go every 20 minutes. You can buy a 1-day vaporetto pass with a one-way airport bus for 32 EUR. Buy when you arrive, either at the desk or the automat at the bus stop outside the airport. Bus is a low floor for easy boarding with luggage. Bus is 10 EUR one-way iff you don't want a vaporetto ticket, but if you want to see as much of Venice as you can in one day, you should get a day pass for 25 EUR. I assume you are taking a train out the next day.

If not the bus, is there a taxi (not water taxi) service? Is it expensive?

About 30 EUR and it only gets you Piazzale Rome where you still need a vaporetto ticket

I've been looking into ATVO vs ACTV because our son and family will be arriving to the Venice airport in December. As I understand it, the ATVO is a coach bus, a little bit faster just because it makes no stops, and luggage goes underneath. If it were two adults, I'd say to take the ACTV, but with little kids I thought it would be a pain to have to keep all their bags with them. Everything is more expensive than these two buses, some ways are MUCH more expensive.

You can get the CheBateo? app which has the vaporetto schedules. It's about 18 minutes to Rialto Mercato from Piazzale Roma where the bus will drop you off. Rome2Rio says it's a 21 minute walk, which may nor may not be a good idea for you ---- saves money unless you are going to buy the vaporetto pass anyway.

Another option is to take the Alilaguna water bus directly from Marco Polo to Rialto. It's a bit more expensive (~15 euro) but there's something special about entering the lagoon and Venice by water. The Orange Line (Linea Arancio) stops at Rialto on the San Marco side of the canal.

But then getting across the Rialto Bridge to the Mercato side with luggage is quite a challenge. That is not something I would want to take on.