I've never been a fan of people that "leash" their kids. But when my son was a toddler, I had one of those plush animal backpacks that had a leash attached to the back... Could be damn simple to put a leash on the life jacket.
Also, the life jackets that aren't the traditional ones that are still USCG approved might be a great option for comfort.
https://www.coleman.com/puddle-jumpe...000001298.html