I think you need to tackle your DH before your children. If they see that Daddy can do whatever he wants no matter what Mammy says then why would they see a need to listen to you or do as you request?

Unplug the Wii and hide the controllers. Ban DVDs. Don't buy in snacky foods so DH can't give them out. He has to listen to you so your children will too.
I agree about the Wii - getting a child's senses hyped up just before bedtime is not going to help at all with a calm bedtime routine.

While you want a gentle parenting approach, a six year can certainly have plenty of boundaries set for what is acceptable at certain times of the day. Set specific limits - 10 minutes of reading in bed before lights out (eg), then lights out. Mum/dad stays for (x) amount of time then it is time to go to sleep.....this is still gentle parenting in my idea of things.