David Wells says Marshawn Lynch is dead wrong for cussing at kids during his football camp last month ... telling TMZ Sports, "You gotta be more a little more professional than that."
We broke the story, Marshawn was out at his Fam 1st Family camp in the Seattle area on July 28 ... when several kids -- some as young as 9 years old -- broke down crying because he swore at them while making them run laps.
In video obtained by TMZ Sports, you can hear Marshawn admitting to using the F-word and the N-word toward the kids in an intense confrontation with some of their parents.