The San Quentin 24 looks like a good spec (and a great looking bike too). Regards the gearing, what have you noticed? 30T x 11-46 gives a low gear of about 16 gear inches which is really low - ie moving at a snail's pace where balance is the limiting factor. You know your kid and terrain the best but I'd say no need to go any lower than that. And then a high gear of ~67 gear inches should be plenty fast for an 8 year old on the flat. The Microshift Advent stuff gets pretty good reviews for kids bikes - there's a few threads on this forum. There are fairly big jumps between gears on the lower part of the cassette, ie 6 through 9, but I don't really think a kid will notice or care. Getting them to shift at all can be a challenge 
If you're looking for things to upgrade I'd probably go for tires first. The stock 2.6" tires will be fine but I don't think that plus width adds much performance benefit for a welter weight kid however does add extra weight (and they're wire bead too). So I'd pick up some high spec folding bead tires in a 2.2" - 2.4" size. Set those up tubeless and you'll save a chunk of rotational weight. Schwalbe Rocket Ron 2.35" are a great light weight trail tire. Or a Maxxis Rekon 2.2" back and DHR II 2.3" front is what my kids run at 24". There's a 24 x 2.4" DHF as well for chunkier trails. Although for tires, as with many things at present, your mileage may vary regards availability - so good job on getting the Marin for your kid.
If you're looking for things to upgrade I'd probably go for tires first. The stock 2.6" tires will be fine but I don't think that plus width adds much performance benefit for a welter weight kid however does add extra weight (and they're wire bead too). So I'd pick up some high spec folding bead tires in a 2.2" - 2.4" size. Set those up tubeless and you'll save a chunk of rotational weight. Schwalbe Rocket Ron 2.35" are a great light weight trail tire. Or a Maxxis Rekon 2.2" back and DHR II 2.3" front is what my kids run at 24". There's a 24 x 2.4" DHF as well for chunkier trails. Although for tires, as with many things at present, your mileage may vary regards availability - so good job on getting the Marin for your kid.
