p.s. one last thing. I have a 12 year old too and the GUILT I feel at not being able to do so much with him is crazy!! Sometimes when I am exhausted, I let him have a friend stay over on a friday or saturday (sometimes a different friend each night if I'm really tired), make sure there's pizza, treats, etc and they just get on with entertaining themselves. Plus they think you are a fab mum! This means I get to lol about without worrrying about my son. Plus, they get lots of 'return' visit to their friends homes.
Also, I have hooked up with other rugby/football/cricket mums to share out the lifts - again my son doesn't see this as part of my MS, just being a canny mum...
Finally, if I have forgotten milk or something from the shops (cog fog!), a 50p bribe works wonders and my son is off on his bike before I can close my purse. Kids, eh?!
One other thing - kids, especially at that age don't need you quite as much. All they really want is for you to be 'there', to offload to and chat with. I try my best to keep up to date with all the goings-on in his life, his friends names (even if I have to write them down in my Blackberry, lol), what tests he's got in school, homework etc. If they realise you are interested, i think that can count for more than blowing all your energy on a day out!