Honestly, a simple card or phone call means a lot. So many people shy away from you when your child/children die that just letting them know you are thinking of them and their child, acknowledging their baby's existance, makes a great impact. And maybe call them to talk/listen in a week or so and then in a month or so, for no other reason than to find out how they are doing and to allow them the opportunity to tell you about their baby if they choose.
If you really want to send them something, how about a plant or seedling of some kind...something that they will be able to nurture, watch grow and associate with their child's life.
Megan...mommy to 3 boys...Angels Grayson and Zane and little Graham!