If you hunt where shots are 150yds max, you picked a caliber that can do 800yds all day!
Nothing inherently wrong with that, but seems like overkill for a 7 y/o?
Then again, of its shot from a rest, then finding a caliber which is efficient out of a short barrel isnt a big deal.
Bigger bullets sound attractive at the risk of possibly more felt recoil, as you want more impact to reduce chances of a bad shot not making a kill.
300 whisper? Id say 450 bushmaster but that also seems like overkill. More like a 12ga slug really.
Anywho, just spitballing.