24" would be really tight and I do not think it would be at all comfortable. My main concern would be the heat accumulation in such a tight space at the highest spot in the van.
Sleeping lengthwise and side by side - Min 32" to 36" above the mattress with the kids sleeping lengthwise in the van. They would have to crawl in and scoot around to get into bed.
Sleeping across and set up as a single, maybe you could get by with 30" to 32" in height above the mattress since they could roll out of the bunk.
Just my thoughts.
Beastie 3: 2002 7.3 EB Cargo: Agile TTB, CCV Mid Top, Custom Walk Through, Lots of stuff added. BlingMyRig.com