
That bag probably has enough air volume to last someone around 5 minutes (give or take, probably give since that’s based on adult respiratory rates, and that’s a kid) before noticeable symptoms of hypercapnia (too much CO2, which will become a problem before oxygen deprivation does) start setting in.
Depending on how wide the river is, that’s not actually too much of an issue.


As long as the river crossing doesn’t take (significantly) longer than around 5-10 minutes, there’s probably enough air volume in that bag for CO2 levels (which are actually the more accute problem in an airtight space, not oxygen) to remain acceptably low