@Rex
People walking slowly in a marked bike lane are, as above, more often than not a Sunday-morning-families-out kind of issue. On most recreational paths.
IMO your coworker was off base to be horribly offended by being asked to pay attention to the bike lane. The rider was off base if they were not being alert for such a problem. If the child did not get hit by the bike it appears that they were doing so. I wasn’t there so perhaps the exchange was a bit testy.
If that was a problem I suggest your coworker not travel in Amsterdam, as in the Netherlands. They have wide, clearly marked bike lanes because much of the city travels on bike, commuting where we take cars. They will totally curse you out if you don’t treat those as real transport lanes and pay attention to whee you are walking. Given that there can easily be 20 bikes in a tight group at intersections, often over cobblestones that are more slip-prone for bike wheels, the riders have a point.