I am 46 but you still may not like my opinion
I understand your need to vent and I'm glad it's helping you. You seem over the top frustrated at this point and I'm wondering if you are feeling personally threatened as of late.
In other words if these were someone else's kids and you heard about them you might have the same opinion about them, but not feel so compelled to take action? Is it to the point now where they have become a threat to your marriage? Your financial stability? Something else? His daughter:
She is an adult so there is no legal obligation to do anything unless she is a full-time student and your husband's divorce agreement includes payment for college, etc. until she finishes.
Arranging a meeting behind your husband's back sounds like a bad plan all around, especially with your current anger towards her. This will likely force future obligations to see her if things get physical and police are involved. His son:
Since your husband is fulfilling his child support obligations and his son is reluctant to see him when he doesn't get his way, your husband is doing what he should be at this point.
My kids are teens and would rather spend all weekends with me (where they have their own room and friends, etc.) but I don't back down on visitation as I feel it's important they are part of both of their parents' lives. The only exceptions are agreed upon by my ex and I and involve 'special' days, school dances, homecoming, etc. where we may switch weekends around.
It's unfortunate your husband's ex fails to see the harm she causes by encouraging them to be upset with their dad.
I think the disturbing factor here is that you alluded to the possibility that his son may be delayed. I am sure it is hard enough for him having 'friends' call him dumbass...he could probably really use a good mentor, father figure to guide him and encourage him to pursue something that will suit him in the future. He probably gets looked down on enough already so 'I guess you could be a garbage man' probably won't help him any to hear.
It is very unfortunate your husband's ex tries to hurt your husband with the negative talk...in the end, it will (and has) hurt the kids more than anyone else...my ex did this with my eldest...trust me, I get how frustrating it can be. his ex:
If you talk to anyone at all, I would think it should be her. If I were accused of having an affair with a married man and it were completely false I would be inclined to send her an email letting her know that you will not put up with her libel/slander and that it is unfortunate she would resort to this as in addition to being completely false is hurting her own children and their relationship with their father in the process.
What a ding-dong she is... Your husband:
When you said it is affecting him a lot more than you, my first thought was "I'm sure it is"....these are his children and although you may not like their behavior, it is hard not to feel responsibility, guilt, hurt, frustration when their choices, behavior are not what you planned when you held those babies in your arms....
my parents did the tough love thing with my brother when he was 19 and in his own fantasy world...they totally cut him off...he had to quit university (he was close to failing out anyway and told my parents he was transferring to a University in Florida...on their dime of course...lol) He did go, but got a full time job digging ditches for gas/water lines....in the Florida heat...until he could apply as an in-state student....got loans and graduated with straight A's...accepted to a prestigious medical school and a very successful surgeon with a private practice now....tough love, but never give up on your kids....
My son also wanted to tell me how to spend my hard earned income on him and not follow the rules of our home...his dad took him in immediately and within a few months kicked him out for the same reasons....now he works full-time in a warehouse and has his own apartment....I do pay 1/2 his rent but said it's for 1 year only to get him on his feet...he is now looking into full-time university in the fall, student loans and asked me to review an admissions letter he drafted....I will support him/help him but only if he takes the initiative to help himself...he needed to learn by doing...that he wants more than a 'job'...he wants a career that he loves...
I don't know where your husband is at with all of this, but just because they are being immature brats doesn't mean he's no longer their father....I do agree that he should not give in to their ridiculous demands...but instead of totally shutting them out, I think he could just be very consistent with his guidance. There are many things young adults need that are free....how to set up a budget...ie....if you get a full-time job here at $12 an hour...car/rent/utilities/etc....here's what you could afford...advice on credit cards....get them if you can to establish credit...only use them for $20 a month or so to be sure you can pay them off each month so you will establish good credit.
To cut them off will only make them worse and feel abandoned (which they should feel if their own parent won't speak to them)...I know it will make you feel better but the long term affects on your husband and his kids will likely rear its ugly head at some point.