The safest PFD for your kids is the one they will wear, so having it fit comfortably is essential. All certified PFD's come stamped with the suggested size range on them. USA and Canadian certifications are now "harmonized" for all of north america, so sizing and buoyancy are fairly consistent across all brands. "Puddle Jumper" or water wing style pfd's,(pictured), are now certified as well, and are a great option for swimming at the beach or pool, but should not be used being towed behind a boat/jetskiing etc.
Suggested Harmonized size range approx.
INFANT - up to 30lbs (has mandatory head flap)
CHILD -30-50LBS (no head flap required)
YOUTH(small) -50-75LBS (no head flap required)
YOUTH(large) -65-90LBS (no head flap required)
JUNIOR/TEEN -75-125LBS (no head flap required)
"Competition" or "Impact" vests are not certified and are not considered pfd's, they do not have enough buoyancy to keep your head above water.