This may or may not be stated in the earlier chapters, but what and when is the typical school day time schedule for the DP class? In your universe, are the days longer than 24 hours, which necessitates regressed Littles having naps and early bedtimes?
Littleforbig on Amazon. Cloth backed and plastic backed. Good selection of babyish prints too if that's what you're after. They have plain plastic backed as well. They absorb a lot with out 'sweating' through the cloth backing. If you like a really thick look and feel get some boosters as well. There is a brand called Rearz that also sells on Amazon. Be sure to check out their diapers too but they boosters on there that are great.
It really depends upon you want. Pull-up or diaper. Clothbacked or plastic backed. How absorbent you want it. Decorated? Cost. Size. There are lots of questions.