Does the EASA/FAA require additional life vests, and life vests for infants/children?
From the FAA's Flotation Equipment for In-Lap Children InFO (Information for Operators):

The FAA recommends:


  Consider installing flotation equipment designed specifically for in-lap children.


(Emphasis mine.)

We also find:


  If an air carrier provides flotation equipment for in-lap children that is located or operates
  differently from the typical flotation equipment for other occupants, then additional information
  regarding that special equipment would be required in the briefing given to passengers.


Which means it will briefed to the passengers if it is present. (Too many if's.)

For the FAA, there is no requirement to have multi-sized flotation devices. Remember, seat cushions work well, and infants aren't good swimmers, but they also don't panic in water (maybe not all of them).




  A reflex called the bradycardic response makes babies [up to 6 months] hold their breath and open their eyes when submerged in water, says Jeffrey Wagener, a pediatric pulmonologist in Colorado. (Source)


Cute video: Baby's first swim - underwater!