We have a 5 and 3 year old who will be wearing PFDs full time on our boat. My wife is doing a pretty thorough analysis of the best brand/model to keep kids comfy and safe when on/in the water for a full day.
I told her I’d reach out to the CF.com community to get info from the fishing/boating families.
Thanks
2014 Key West 203DFS
1987 Landau
I always had my son in a zip up vest style Mustang PFD up until recently. Their smaller PFDs have a strap for the crotch to prevent the vest from slipping upward.
Iain Pelto
Sea Hunt Triton 160 w/ 90 ETEC “JB3”
Native Manta Ray 14
I would suggest the neoprene ones for comfort. My kids like them better. Also, if you are up on Saturday mornings, you can find GREAT deals on them at yard sales when families kids outgrow them. I frequently pick up nice ones for $1 or $2. However, I will warn you - there are NONE to be had East of the Cooper (that is where I go!) 
I definitely suggest the ones with the strap that goes between the kids legs. That way even if they have their arms straight above their head the life jacket can’t slip off.
I usually keep an extra kids size lifejacket on board for friends or a spare if one gets ripped.
I just use a regular old life vest on my son. He complained about it the first couple trips we went on, but he’s used to it now. Make sure you weigh them before you buy a PFD. The weight minimum/maximum is on the inside of the vest.
*Edit: If you wear your vest all the time, they won’t complain as much about wearing theirs all the time. 
Semper Fi
18’ Sterling
115 Yamaha
Big Ugly Homemade Blue Push Pole
Our kids like these:
They are good for little ones, especially those just learning to swim. Seem to be more comfortable than most jacket style and are pretty secure. They are Type V, so only meet PFD requirements if worn but obviously the kiddos should be wearing them all the time anyway.
I think they are called Puddle Jumpers, we got them at Target. Also have some like Hoof’s pic above that we use sometimes.
Angler 204 FX
Yamaha 150