Day trip to Cinque de Terres and Venice FROM Milan

Both Venice and CT are simply too far to be considered day trips. You are looking at 5-6 hours train travel total for each. Add to that time on each end to get to/from stations. That's most of your day spent on a train.

What time of year is this?

Not sure where you are looking but last Fast Train from Venice to Milan leaves at 7:48 PM- arrives Milan 10:15 PM.
https://www.trenitalia.com/content/tcom/en.html

Milan is too far for a day trip to the Cinque Terre as is Venice. And with children so young, a daunting journey at best. Traveling with 23 is also a challenge! You need to relocate to a place near one or the other, one at a time.

Both CT and Venice are just too far for a day trip. If you REALLY want to see CT or Venice (not both), take those 3 days and spend them at ONE of these locations.

A trip to the alps is more reasonable, but still a very long day and you’ll be spending most of the day in a train. I would really question if that’s really how you want to spend your time. More info on the rest of your itinerary would be helpful. It sounds like you are trying to do WAY to much in the time you have.

Sorry but I can't imagine doing long day trips with 2 young children. We took our daughter to Italy when she was 15 mo, but my husband remained with her in Rome while his parents and I did a long day trip to Pompeii.

A slightly better day trip could be Milan to Santa Margherita Ligure. There are direct IC trains that take just over 2 hours from Milan.
8:05-10:14, return 9:10-11:17

Still a bit too much for a day trip with so many people to herd. IMO

Throwing my vote in with the others: Venice and the CT are too far for day trips plus both of these destinations are lousy places to try and herd a group of 13 on/off packed trains (CT) and boats (CT and Venice). Trying to stay together would be an exercise in frustration. As well, both destinations are sort of weather dependent for best results: wouldn't be fun if the few hours you had to explore were in pouring rain.

Can't answer for the Alps.

The suggestion for Santa Maguerita is a good one. I am a bit curious why you're staying in Milan if you want to spend 3 days going elsewhere?

What came to mind upon reading the 'plan', is this quote from the movie The bridge on the River Kwai:

"Madness....madness"

Is this 3 consecutive days of travel? If so, I suspect it will be a tired, if not grumpy group, by day 3.

kathryn, if you can share much more detail about this plan and this group, we can offer suggestions. The most prudent thing would be to enjoy Milan or the nearby lake area. OR, go to Venice, stay there, then return for your onward flight.

I just responded to your Venice vs. Cinque Terre question. I agree with all who have responded here that you have too many people and too little time.

Unless your group plans to separate, with some doing their own thing during the day, then getting back together for dinner, you will travel at the pace of the slowest person in the group.

hey hey kathryn
as joe32F says "madness madness!!"
too many people to be shuffling around. are all the adults over 30? traveling to venice is too far for a day trip, more time on a train than seeing the sites, the toddlers will go into tired/cranky mode. you thinking you have a full day is way off going that far. why didn't you fly into venice, so much easier
going to lago di como (lake como) will take time since you will need to transfer trains and spending more time sitting, and may need a few taxis with your group.
this sounds like more adult activities than having to cater to toddlers now. are you flying from USA and plan day trip next day? remember jet lag, checking into hotels after 3pm, will your group need down time from each other?
then you add the alps within the 3 days, don't know if you are a for real poster and asking a not intelligent questions.
aloha