D.K.
I have to say kudos to you wanting your step children involved in your life! That is great.
A lot depends on what they agreed upon during the divorce as far as visitation goes.
He can fight it very easily that they come every other holiday, sharing expenses with the mom, and so many weeks in the summer.
I can say being on the flip side, my ex moved back to Texas. He has asked if maybe my daughter would come visit him this summer in Texas. She hasn't flown since she was 18 mos, my son isn't old enough to fly without a parent so she would be going alone. I asked her if she wanted to fly to see her daddy and when she found out I wasn't going or her brother she was very aprehensive about it and said no. I personally don't like the idea of her travelling alone on a plane her first time to see him. I haven't said that to her but to be honest I am letting her decide. I have always told my ex that it isn't up for us to dictate visitation, it is up to the kids. Even with my personal reservations, if she said tomorrow she wanted to go then I would send her.
When they are older, I think they will want to spend weeks in Texas with him, holidays and so on. Per the court I have to help pay for that expense WHICH sucks as he moved I didn't but courts look at what is best for the kids not the parents.
It has to be about them.
If they don't want to be away from home that long, enforcing it could cause a lot of resentment. A 13 year old should be able to make those kind of decisions herself as well as the 9 year old. His ex should be open to them flying and doing whatever visitation they want however. He can even enforce that she splits the airfare too! It is about what is best for the kids and let the courts decide.