Unless the client insisted otherwise, I would strongly encourage them to take photos of parents holding the infant. That is pretty much the most natural pose in real life for infants: being held (or if not, swaddled etc. which is just a less photogenic version of automatically holding). Or wearing the baby in an appropriate baby wearing device.
Putting them by themselves is inevitably weird, because babies don't just hang out by themselves in big white rooms, etc.
You'll get a natural look by replicating what naturally happens.