Agree that it just takes a ton of reinforcement. But they are probably tonguing way too heavy and just waiting to hear the beat instead of really feeling/counting it. SOMETIMES I tell my horns you have to "swallow/gulp" the downbeat but I would really caution you on doing that because if they are young players, it will do more harm than good. In addition, quicker, shorter breaths (that don't occur very often) will help.