Congratulations on your new granddaughters!
When I was in a guild we made quilts for the NICU. We used all cotton fabric, thread and batting (even though we used poly bating for other donation quilts). All cotton was the rule for the NICU.
The hot water wash is important as Bug Lady says, and the quilts will be washed in hot water very frequently. For the first wash, I would use lots of water and Synthrapol or colour catchers. Make sure you quilt densely so that the quilts can stand up to regular trips through industrial washers, but don't worry about the quilts lasting forever - you will need to make them bigger ones for tummy time soon enough! Even if the colour runs a bit, you will still be covering your sweeties in love, and that's what matters.