May I say first of all how admirable that you have taken on looking after these two grandchildren especially with a physical challenge to deal with aswell. Is the oldest who has ADHD ,on any meds , I ask this as sometimes the meds given for this disorder can cause some behavioral issues ,he is also the eldest and very often they instigate problems from feelings of jealousy ...How about the younger one , he is diagnosed PTSD is he on any meds? I am sure there must be programmes in your area to get help for them you would need to put into the search engine the area /town where you live .There are also good books out there that may help you ..one I have found very helpful is' SOS help for parents' by Lynn Clarke. In your case I would also use the 'taking away method, if they behave badly , take privileges away, no TV no Computer, no games remove something they enjoy , be consistant they only get the privilege back when they behave.. Seperate them up in the house if they egg each other on . Are they involved in any sport children thrive on outside games and exercise...Not easy I know but be firm and it will get better .good luck .