We have done the same search and not found a PFD
with harness. Ended up getting a harness for underway or on deck
at night. and a PFD
for other times.
There has been a similar discussion here: Children and Offshore life jacket/harness Safety
There seem to be plenty of good harnesses on the market. Make sure it has a crotch strap, kids
slip straight out otherwise.
The littlest admiral has this one: Baltic - Junior Safety Harness - 0125
forget where I actually bought it. He is 25kg and grew to 10 ft tall the first time he strapped on and went to the bow by himself underway. Has no complaints about wearing it but gives me grief if I'm not tethered on as well.