Jackson could care less about the kids he knows and is familiar with (my little 6yr old brother and his close friends, cousins, and my little sister). They can run, scream, whatever, he doesn't care and likes to join in on the fun.
Strange kids, however, if they make fast/quick movements, he will sometimes do the same barking thing. He's definitely an alert dog. I can tell the difference between his play barks and alert barks. If he's just playing, I let him, he has fun... but the alert thing can be annoying. I use a squirt bottle. It may sound cruel but it's just a few squirts at him and he stops. I tell him "no bark" every time and he listens. He doesn't bark at kids if were outside or in a store or public place, it's only in our house.