Help please

:gen147: My dog Sam who in every way is the most loveable pooch in whole wide world cuddles ,kisses, following me like a shadow but he has 1 bad habit i can't seem to get him out of (he was a rescue dog) and that is mouthing does it all the time when i'm putting his lead on if he wants something but what scares me the most to the point i get anxious when we are out is i think he Hates kids if the run by him he tries to jump up on them i see them coming and i pull him in close to me with his lead but im terrified he tries to mouth 1 of the kids and the parent thinks he has went to bite them and calls police we are up the park every night were he plays with 7 other dogs all off their leads no trouble at all ...also when it's just me and him with his ball thrower thing again no issues but when im at our house and he is back on lead and they run by or are screaming near him he goes beserk with his hackles up and barking real loud it must scare them.....any suggestion greatly appreciated as i said if you saw him in the house and up the park you would swear blind i had 2 dogs thanks in advance xxxxx

Thanks i think i will have to give it a go fingers crossed