While i really, really, like the grendel i dont think its a very good youth cartridge, unless said youth is experienced at taking game (not just shooting targets). The penetration is good to excelent, but there is little shock do to the low velocity, wound channels tend to be smaller then any of the larger cartridges (and smaller than the fast 22s that only penetrate 10"s or so). That said, the round is excellent as long as bullets are placed properly, and your willing to track, or you break bones. The only production guns im aware of in the Grendel are the Howa1500 minis, I have a regular howa in 6.5 creedmoor and would not hesitate to buy one in the Grendel if i could find one.
The creedmoor, Swede, and .260 are all pretty much the same round when were talking about taking game. One maybe 100fps faster with a given bullet weight than the others but it wont matter too much. I havent shoot any game with my 6.5, but Ive seen a ton (and taken quite a bit myself) shot with the .243, the extra 25 grains and larger frontal area would make the 6.5s quite a bit more effective than the .243, this is considering keeping velocity in the 3K-2800 range, which is where i like to launch most of my bullets. This would be my choice, as you can get pretty much ANY bolt rifle you want in one of the short action 6.5s. Theres a 12" lop howa stock on ebay Ive looked at a few times just to have for the future, as i said else where while its effective im not a huge fan of the .243 as a beginners rifle either but i think the 6.5s should be excellent.