I understand but, well, it's not actually OK if only one of them is OK with it, is it? So that's irrelevant.
Also, kids may tell you that they are OK with it but secretly not be. Kids do these sorts of things for many reasons, perhaps wanting the family to feel like one family and therefore being OK to make a sacrifice. But it's not OK.
Even if the kids have requested it, you as the adult still have to make things equal. You can't have some getting more than others and then claiming to be OK with it because they feel they should, or that it will be better that way. That's messed up.
Even if the kids tell you it's OK, it's not - what does it do to your blended family dynamics? If you are talking about becoming one family, then when it comes to gifts from you and your partner, you need them all on equal footing.
If they get presents from different people at other times, later in the day, whatever - that's fine - but you are talking about your first Christmas morning together as one family. Make it equal and start as you mean to go on.
Just pool everything and split 5 ways. If you don't want to do that, that's fine, but don't blend the family on Christmas morning.