The first two reported timestamps are 16:00 and 16:30 which both fall conveniently on the half hour. This leads me to conclude that these are guesses, and could be either early or late. It seems most likely that the mother was not exactly aware of the time the child left the house. There is an incentive to minimise (to avoid it looking like a failure of supervision).

What we are certain of is the 16:44 call to the EMS, so you're right, 134 minutes is the lower bound. However, it's not unreasonable to assume that the parents did not immediately call on noticing the child missing, so the gap between 'noticing missing' and 'calling EMS' is real and non-zero.

The gap between 'leaving the house' and 'noticing missing' is something I'm less clear on: how did the parents 'notice' the child was missing when he was knowingly allowed to leave the house earlier? There's still a non-zero gap between 'last sighting' and 'realisation that he's missing' (i.e. he definitely wasn't missing at ~16:00, when the mother saw him leaving the house... what changed so that the parents believed he was missing at ~16:30? Was he supposed to check in with Dad and missed the check-in, etc?)

Either way, a difference of ±10% doesn't really make that much impact in interpreting the results.