Generally 7 and 13 year olds do not have a choice with regard to following custody orders and they should be enforced.
If I was being denied access from children this age, I'd be in court seeking a revised access order based on the ongoing contempt.
One thing you also may want to request from a court remedy is reunification counselling so that the children can recognize the incorrect behaviors of your ex and start to work with you on fixing your relationship.
Unfortunately, there's no quick fix here....it involves a legal battle.
I'm very sorry for what you're going through. I have nothing but loathing for parents who unfairly deny access to the ex spouse. Your children have a right to have a relationship with you. I know its tough both financially and emotionally but if you can, fight for them.
Jenn: I think 13 is a difficult age and a 7 yr. old, being young, is going to believe most of what they hear. When the younger kids get older, they too may realize that their dad's behavior is incorrect just as your oldest does. My daughter was 18 when ex left and although they had a good relationship growing up, as she entered here early 20s, she realized the mistakes he had made - she actually feels guilty because he spoiled her and spent too much money on her. What I would suggested is, when your other kids get older that you try to meet with them and tell them that their dad was not truthful and I agree with the other poster about trying counseling. I don't think the kids should be forced to go anywhere. They will only become angry if they are. They also may have friends they are spending time with where they live that are taking up a lot of their time (not that they shouldn't be spending time with you, also). Judges hate this sort of alienation behavior and urge former couples to be civil for the good of the children. Maybe a neutral person (an Aunt or someone they are close to) could talk to the other kids someday and tell them the truth. Best wishes.
Please do not follow the suggestion to involve your children any further in your divorce.
Children should never be involved in adult issues between their parents. That is what your ex is doing now and its the reason why your kids are struggling to have a relationship with you.
Please never stoop to that level. I have a deep disrespect for people who try to sway their children to their side by bashing the other parent or giving them their "perspective of reality."
Children...regardless of age...have zero business being involved in adult matters. They will eventually form their own conclusions and should not be influenced unduly by either parent.
I hope you can find a good lawyer and a good counselor without having to spend a ton of money on costs.