A couple is entitled to plan the wedding they want to have. If they wish to elope, that is not wrong. It's their wedding and they are free to plan it however they see fit.
That being said, when a couple chooses to elope they also need to accept the fact that loved ones will likely be sad/mad/hurt about it.
It's a Catch-22. The couple is not wrong to want to elope by themselves, but their loved ones are not wrong to feel hurt that they were excluded.
If you are the bride- or groom-to-be in this situation, then think about how this choice will affect your children. You are still entitled to elope if that's what you want, but don't be surprised or offended if your kids hold a grudge against you for it.
If you are one of the children in this situation ... you are not wrong to feel hurt, and it would not be out of line to tell your mother/father that you feel hurt that you were not invited, but you also need to accept the fact that this is their wedding and they get to plan it however they want. And if they return from vacation announcing that they eloped, you can complain and cry and argue all you'd like about it, but that won't change the fact that they are now married and the deed is done. So arguing about it might certainly feel cathartic, but it won't make any difference in the long run. And you may not get the apology or remorse you are seeking from them, so keep that in mind as well if you decide to pick a fight over this.
Also, just because the plan plans a surprise elopement for his fiancee doesn't mean that she's obligated to go along with it. She is free to say NO if he pops a surprise wedding on her and she doesn't want to do it. If she goes along with his plan knowing that it'll hurt her daughters, then that's her own fault, not his.
Also, can a "surprise wedding" really happen in a place that doesn't allow for instantaneous marriage licenses, like Las Vegas? In most places in the U.S., both members of the couple are required to apply for the marriage license at least three business days in advance of the wedding. In the vast majority of cases, there's no way a man could plan a "surprise wedding" for his fiancee, where she shows up to their vacation spot and he's waiting there in a tuxedo with a wedding gown on a hanger for her and an officiant waiting to perform the ceremony right then and there.