@Toney198 sounds like we hunt in similar terrain. My daughter (13) and I sit in double ladder stands together, and we are always near thick cover like you mention. This is her 5th season hunting with me.
In my experience, very few deer drop where they stand with vital shots. I have had some do, but most run some. I am sure there are hunters that have shot deer with many more cartridges than I have, but my experience is limited to killing with 44 mag, 223, 243, 300 blackout, 30/30, 6.5 grendel, 7/08, 308, and 270. I have had them drop where they stood with the 223, 300 blackout, and 243. I have had them run a few hundred yards with the 7/08, and run some with the 270 and others.
I'm not a fan at all of neck shots - 2 bad personal experiences - so I won't take them.
I think the 6.5 grendel is a perfect cartridge for youth, and the 300 blackout is a close second.
The 2 deer we shot (her 1 and myself 1) both ran about 40 to 50 yards, and both were behind the shoulder shots. Her buck bled everywhere - he had it coming out of his mouth. My buck didn't bleed a ton - both were shot with handloaded 123 sst bullets.
I've got a box of 123 eldms. I'm going to load up some this weekend with 8208 and try those for the rest of our season. I'd like to see what they do in comparison to the sst bullets.
The Howa mini we have has a 20" semi-heavy barrel. What length barrel do you have on the AR you built your son?