Because you only know one of the children.
I'm sure this isn't a gift grab, I'm sure this is just something they worked out as some sort of convenience to someone...either the parents or some co-relatives/friends, or perhaps even the priest (if they both wanted the same day, for instance, and it was really important to each of them).
There is absolutely NO chance that I would feel, even for one minute, like I even had to get a card for the kid of the people I don't know, let alone a present. And I don't think you should have to either.
And if it IS a gift grab, there's NO reason for you to opt into it!