you poor thing. I have a small idea what you're going through because I moved recently and since the move our cat has had "issues".
I think Fletch's idea of taking them to the loo at night is a great idea and hopefully they can get in the habit himself after a while.
I have another couple of suggestions for Mr 7. By his age he should be capable of using the washing machine, hanging sheets out and making the bed so maybe you can take the pull-ups back off and make him clean his own sheets and re-make his bed. (with summer here doonas aren't such an issue) Maybe having such a tedious consequence will make the loo seem more enticing.
Also if he doesn't do it at school he must realise that its socially unacceptable so is there any way you could get him to spend a really extended period of time with some other kids ie with the school holidays coming up you could arrange a kid swap with a really close family friend or family member; thier kids come to stay for a while and then he goes to stay with them and then you have some differant children over and then go camping. Maybe if you could arrange a fun way that he could be in a social setting constantly for a week or two he might by the end of it prefer to be cleaner or be in the habit of being cleaner.
I hope you sort something out soon.
Is there a school counsellor at his school? Maybe they can help with strategies.
Also if he does go for say a week with no accidents why not take him somewhere really special with just him. I think that toys and star charts are great rewards but special time with your parents is even better and if Mr 4 knows why he might try to win a 'date' as well.

just a quick edit to add some extra thoughts - I think that if you get him to wash his sheets and remake his bed you need to make it clear that it's not a punishment but a consequence IYKWIM. Also if I was going to to get him to spend lots of time in a social setting I would be sneaky and not mention that its a cunning plot to try and stop him pooing so often.