It is horribly disappointing to open a Christmas gift and not be able to use it because you do not have the batteries or memory card that it requires. Even worse when a child can't play with their new toys.
Be sure to buy these needed items and include them with the gifts. If you can do it without ruining the packaging, put the batteries and memory cards in the gifts. If not, they make a great stocking stuffer.
This is such a simple thing, but it is an act of consideration and makes for a joyful holiday.