Telling your kids some of the gifts are "from St. Nicholas" is true, in a similar way that saying all of our gifts are ultimately from God is true.
If we give gifts in imitation of the example of the good bishop of Myra, we are giving gifts "from St. Nicholas." As the kids get older they'll learn that the saint's example is one they can imitate, too.
Actually, I had it in my head that I would buy very nice gifts for my children on their birthday to truly celebrate them, but tiny meaningless things for Christmas so they can concentrate on the reason for the season. To be fair, mine would have enough aunts and uncles that they won't miss out.
I think that's a good way to do it!
The ideal for Christmas, I think is to teach kids to be generous themselves in imitation of God's generosity to us. So you want to find a way to shift the attention away from themselves on outwards in charity.
