Depends if you want an approved design, or one you think will work?
very good advice from the US Coastguard:
"The US Coast Guard does not recommend taking newborns below 18 pounds onboard a recreational boat unless the parent is able to test their newborns out in a PFD, sized for infants, in a swimming pool. Testing is the only way to ensure that the device will float the infant with his/her head out of the water. Unless you know the PFD you have works for your infant, why put the child at any risk.?"
"An infant or child life jacket should have a crotch strap, which helps keep the life jacket on; an oversized float collar, which keeps the head out of the water; and a grab loop, which makes plucking a youngster out of the water easier."