I'm dealing with an explorer right now, which has even less than the regular sized I'd imagine
While it is true the non XL Yukon is bigger than the explorer, with the third row seats in the up position you really don’t have anymore room than the explorer if the kids are bringing stuff with them that needs to go in the back.
Keep in mind with three kids, a denali most likely will have mid row bucket bucket seats, that means one of the three will have to sit in the third row which will take away from cargo space. I have a newphew and two nieces and my mom and I often bring them from where they live, up to the mountains where we live to spend time with us. When I take my Denali xl to go get them, one has to sit in the third row. And having the XL Denali means I have extra room behind the third seat to put all the stuff they bring with them, where the regular wheel base Yukon wouldn’t have that extra room.