I mean, it’s possible he’s gripping a pipe/snorkel of some kind in his hand protruding through the top of the bag itself.

He’d just have to grip it tight and hold it above water as he wades through. Perhaps it’s just deep enough that there’s not a reasonable way for him to hold the kid completely out of the water, but it’s easy enough for him to hold him up just enough that water doesn’t come in over the top of the bag itself.

With the amount of air in the bag to be “full” and relatively pressurized, he’d reliably have several minutes before any issues. It’s not like it’s suctioned around the kid’s face. There’s several minutes of air in there. More than enough time to ford a river with width measured in yards.