Hi Jane
I am not an expert, but I would say back to basics with Charlie! I would not put him on the lead, as like you say, it hypes him up and also he may feel more threatened on the lead, should your little grandsons, get overexcited!
I would use reward training with him and use a word that everyone else must use too such as "calm" or "carfeull" when he suceeds treat him with a titbit, hot dog sausages cut into tiny pieces always work well, perhaps involve your grandsons in this too, so he realises if the saying "calm" and hes calm he will get a treat of them too! Its not impossible to train him to be careful around the boys, Indie is marvelous around my kids, when other peoples kids come into play, she wants to kiss them to death, so we use the "calm" word with her, she soon learnt, that not all children want to be bounced over and then kissed by a cocker! (LOL)
Good luck, let us know the result!