OP, it's pretty straightforward but the heavier the load (backpack, child, etc) the more support you want.. which would lead more towards the higher boots.
Yep....also, if he is carrying a child on his back he really wants to make sure he gets hiking shoes with good treads on them. He def doesn't want to slip on a trail and cause his kid to tumble out as well.
I got lucky and scored a pair of high top boots that I've only worn once(I will need to break them in more before I start backpacking) and a pair of replacement Merrells at REI's garage sale a couple months ago for $35 total. That's $300 worth of shoes....for $35