Would a child still be able to put their head in the water for snorkelling?
My vests are the smallest size and are used by 2 year old children. I would tend to agree with Matthew W's answer but that depends on the size of the children.
This is a Type II PFD, which the PDFMA says "many will turn an unconscious person face-up in the water".
You want that in a PFD, and this one does a good job of it with my kid. As a very strong swimmer, she can overcome the uprighting to swim forward, but not enough to snorkel. (most people backstroke in a PFD)
A Type III "for use in calm water where there is good chance of fast rescue since they will generally not turn an unconscious person face-up" might allow snorkeling, but is not a substitute for good swimming skills. If a little extra buoyancy is all that's needed, a wetsuit and/or pool noodles could work in a CLOSELY SUPERVISED situation.
I can't say with 100% certainty but I would say yes, a child could probably snorkel with this pfd on. Since the cushion on the back is actually a flap (its attached on the top edge only) when going face down in the water its going to pop up. It may be a little tricky but my little ones don't seem to have much trouble swimming on there stomachs with them on. Now, if you come to a stand still the pfd might have a tendency to right them face up if they don't continue to tread water but I think it will work. On a different note: these pfd's are very nice quality, they have a cover over the crotch strap to prevent the shaffing on my little girl that bare nylon straps tend to do a number on the inside of her legs and the neon green is awesome since not a lot if any pfd's are that color so they really stand out in a crowd. You'll be pleased.
Yes they should have no problem snorkeling. It is like any other like vest, with an added head rest. Your child can float on their back without having to hold their head up or strain their neck. When on their stomach they can float and move like any other life vest.