You can explain the purpose of the vows that you and their new step-parent will be making to each other. Share your vow with the child. Explain what it means. Explain that the vow is a promise you make, intending to do your very best to live up to it, always. Then you can invite them to make their own promises to you, your new spouse, and the new family. They don't have to be fancy. Just a sentence or two will do, in the child's own words.

If the child is old enough, they can write them down and read them following the adult's vows. A shy child might choose someone to read it for them. If they're too young to read, but still old enough to understand, the vows can take the form of a series of questions that the minister will ask of them. "Do you promise to ... ?"

It's a lovely, meaningful and personal way to express the reality of the new family to everybody -- guests, parents, and most importantly, the child himself.