I agree with your concerns that a good Type II Infant PFD
will be bulky & restrictive - and might even be ineffective, or worse, actually dangerous.
Babies, typically, have rather large heads, raising their centre of gravity. A PFD may not support an infant upright - in fact may keep him inverted!
I’d suggest you seek out a good Fall Restraint Harness c/w a Shock-absorbing Lanyard, and tether the child to the boat
, when he’s not within the cabin
I’m not talking about a “cheapo” strolling harness, but something that straps under the crotch, around the chest & over the shoulders etc.
I’d also bring the PFD along, for use in the Dinghy
, and in the unlikely case of the ultimate disaster (abandon ship).
BTW: USCG recommends that you test the PFD, on the child, in a pool - due to fit & upright floatation issues.