Total newbie with 4 kids looking for advice on setup

Have tried searching and found some answers but hoping zwift users can point me in the right direction. Looking to get Zwift mainly for my 4 kids aged 10, 11, 14 and 15. They’re all in a triathlon club (3 years) but the oldest wants to start training more seriously and they’re all doing more competitions. We live in a city and getting out on the bikes (especially over winter) isn’t easy. So what turbo trainers should we be looking at? Have found some used smart trainers locally, should I get one and give it a go (could upgrade if well used). Space not a huge problem but it will be put away at times. Smallest bike is a 24” wheel. Looking for the older kids to use it independently. Looking at Zwift membership for kids it seems to be free, is that correct? Also saw mention of a family package which would be an option too. One turbo trainer is a Bkool smart one (have asked for model). any advice gratefully received. Thanks

I have a tacx vortex for my son (10y).

Zwift is free for 16 or younger. Write to support for setup account.

24 inches is going to be difficult to find.

Wahoo Kickr, a wheel-off trainer (and pricey) supports 24" wheels without a wheel block, but I believe any rear wheel off trainer should work with a large enough block under the the front wheel.
Any chance your kids’ triathlon club or other cycling organization in your area has swap meets or advertises used equipment, or better yet might donate?

With different size kids, you might think about a setup that will be easy to adjust seat/bar positions. Concept2 makes a bike ergometer that might work. It transmits wattage, cadence to Zwift, but it’s not “smart” in that resistance doesn’t automatically increase on hills. I’ve been Zwifting with one for 18 months and it works really well. When I get to an uphill, I have to reach down and move a lever to increase the resistance (which increases the watts that I produce).

I’m also new to Zwift but I didn’t see an option for a family package. Is there one?