How to ask your colonial nation to give you territories?

You can only seize province from classic vassals. You can't do it from minor partners (not your question, but thorough answer), or from CN.

You won't be able to get those provinces peacefully.

The only way I can see, would be to hope for rebels to take back the provinces, so you could attack yourself.
On way to do it (you can't of course help rebels in your CN) would be to take a tons of other provinces in this region, either in the direct control of your CN (it will automatically revert to the CN), or be sure to give it to your CN in the peace deal.
You should be able to give a lot of OverExtension to your CN (especially in mexico area), so they will break under rebels.
But of course, that might goes against your primary objective (giving 15 province to your CN so you can get 5 for yourself later). And because you won't have any control over the rebels, it's still random.

I just thought of another possible solution.
Go to war with one of those south west American nation(Co-Bel everyone) with the help of your CN, of course, and loose the war, giving away your CN lands...
(they might not like it).
Might still count as non-violent (for you, after all, you did not fight).

Yet I think abbabab is right, and you ask the wrong question.
You should not worry about those 5 provinces. You will be able to get other CN and the corresponding merchant in due time.

Your colonial nations will always give you 50% of their trade node, which means not having the provinces (but your CN having them) is almost as good as you having it directly. keep in mind that any provinces you get in their area will go to your CN too as soon as that province is self sustaining (that means a colony will be yours and once it is finished, go to your colonial nation).

the only way of preventing a colonial nation getting all your provinces in an area is to have your capital on the same continent, which will prevent CNs from forming on that continent.