Well it sounds like you have a 'fun' parent not a real parentin your ex. If it was a grandparent or an aunty giving them sweets and treats you'd probably grin and bear it a bit, or mention it politely and tactfully.
You have to look at him in that way; he is a desperate man who wants his dc to like and love him. It's sad he feels he has to give them so many treats and stuff to get them to keep visiting. It's sad that he feels he has to give them whatever they ask for rather than cause a fuss.
But if he does stick to the rules and be strict like mum wants then the dc won't feel like coming all the time. They won't want to go for one night to punish him by having a moody strop. And if that was at home with mum they'd have to get on and get over it and there'd be time to sort it out slowly. With dad they only have a few hours and the resentment and punishment and 'being too scared' to go will drag out over several visits, which could equal a month or more.
All the 'fun' parents I know try and make the tiny amount of time they spend with their dc as fun as possible. They want their child to laugh and be happy and wanting to come back and saying how much they love coming to see them. That is the definition of a fun parent.
If you want him to be a real parent - disciplining, boundaries, homework and bedtimes etc then you have to allow him to be by giving him much more parentig responsibility. You need to be uping those contact hours. Then they get to see dad when he's tired, got work the next day, has to get them out the door to school washed fed and homework in hand etc. And he has to pick them up from school that day too and do it all again. You need to share the dentist trips, the phone calls from school when homework not done and grades slipping coz of late nights, and all the other drudge of parenting. Assuming he's willing to do that...
So you either share the parenting a lot more and give him time and the chance to be a real parent
Or you accept that he is only a fun parent and the limitations on parenting that places.
Would you still insist in toothbrushing, sweets only on a wednesday, and early bedtime if you knew you wouldn't be seeing your dc for only one day a week, and that it could be longer that a week (if they come because they got over you telling them off or because it was more fun to stay at their usual home or go out with their mates then go somewhere homework was insisted upon etc)
Don't get angry at your ex, but do feel sorry for him that this is the only way he can be a parent to his children. I wouldn't want the homelife, security and happiness my ds feels with me to be based on bedtime, treats and computer games. I'm here for the long haul.