Pick any more modern round with a proper load for what you want to do with it.... .270 and above... Longer range, flatter trajectory, more knockdown power....
Did you read this part of the OP?
Thinking about going deer hunting with my boy and I'm assuming most of it would be at fairly close range (under 100 yds.) and in the bush.........
And for the record, you do not "knock down" a deer. You drill a hole through it to let the blood out of the places it should be for the deer to remain alive. A 30-30 will accomplish that within the conditions set out by the OP.