I'll be the first to admit that I am not from a blended family, so I may be missing some of the finer subtle points that coming with being a family together. I apologize for that in advance.
#1 I agree with the basic sentiment of the group here. The older ones need to grow up. They are still in emotional competition with a 17 and 18 year old over who gets the most stuff?? Really? Time to grow up! and you and hubby are the ones that need to guide them into maturity. I applaud you for wanting to set some boundaries now so you can enjoy the holidays for years to come.
Question. What does your husband think of what happened last year? What does he think would be a good solution? What ever you decide, you both need to be on the same page.
#2 You never said how long you two have been married. Has this been a more recent marriage? That can make a difference. Honestly, I like your practice of just giving them money for Christmas. The older ones all get the same amount. They decide how to use it. Now you can focus on just being family and having fun together. You also did not mention how many of those 6 children are living at home with you. That can make a difference too.
#3 My mom's side of the family is huge. When I was a kid growing up, we all knew that all the kids/grandkids -- anyone under 18/still in high school got the gifts. Past 18, you were an adult. The adult children got a monetary gift - to use as they chose. The adult grandkids drew names on thanksgiving and bought 1 gift for that cousin. There were just too many of us and as a family we valued being family together on Christmas. Competition over who got what would have killed us as a family.
My suggestion would be for you and hubby to agree on a set amount for each adult child. And let them know in advance that each household (in case they are married or have a significant other.) will get the same amount because you love them all equally. For those that live under your roof...give them the option of that amount or buying gifts. (Let's face it. Even being a mature 24 and holding one check while a 17 and 18 year old are ripping open several packages ..well who wants to be left out of the fun?! Let them choose, with the understanding that you are only spending the given amount.) And with the 17 & 18 year old, do what you feel best. If you able to spend an extra $100 for something then do it guilt free because they are kids and they aren't expected to have the resources or full emotional maturity that someone in their 20's or 30's should have. I would also sit down with your husband and figure out what to do, small but equally, for grandchildren, before you get overwhelmed each year by sheer numbers (you do have 6 children you know.) Maybe a $25 savings bond for college each year per grandchild or something similar. (When you have 20+ grandchildren this will make sense.)
If the older adult children still huff and puff about who got what this year...then maybe you and hubby should say "OK, New family, New tradition." Buy gifts only for the under the roof minors to open on Christmas morning. Spend the money you would have given to the ungrateful adult kids on a special meal together or maybe buy a block of tickets to see a holiday show together. These are your funds, not theirs. Use the funds to bring family togetherness and joy during the holidays. And hubby has to be the key here. His children didn't get their greedy attitudes from under your influence. So they will not think kindly upon you if you are seen as the one making changes and taking their money away. But if you want to help them learn to focus on being family together and not on who gets how much, then hubby has to step up and be leader in creating this new family centered tradition.
Anyway...all IMHO. You and hubby do whatever you both think best and stick to it. Best wishes!