The depth of scoopable litter should be at least 2 inches for young kittens, I think too. But you may have one of those avid ones like to dig to China, lol. Clay granules seem to work best for most cats, but I worried about mine ingesting them when I first started. They started lying and playing in there rather than using it for its original intention! What a mess, especially if it needed to be cleaned! The novelty will wear off eventually, but do keep an eye on them if you do. I think the older they get, clumping clay up to 4 or even 5 inches are best to prevent urine clumps from sticking to the box. If you go to the catinfo.org site, there's an old (blurry) video of how to properly clean/maintain the litter box. There's also a lady on YouTube who is a breeder of Persians and demonstrates proper litter box cleaning in more detail...I think her username is mythicbells(?) I learned a lot from her, she's the queen of squeaky clean litter boxes, and has some good tips.
Another thing you may not have considered important with good litter box and litter is a good scoop! I use a heavy duty metal scooper, but if you need to scoop thru lots of litter, it does feel like a work-out. The good thing is the metal scoop will far outlast your typical plastic ones. But on the other hand, those plastic ones like the litter lifter are what some folks swear by, as it cuts thru litter like butter due to its design, thus cleaning it is a whole lot faster. So, little things do count, especially when you scoop often like you will with new kittens!
You may want to try both out or get opinions from others here, or read product reviews online, etc. Whatever you decide, good luck Bon!