Looking at the design of the product it looks remarkably like an automatic punch with a blade recessed into the handle. The kinds of situation it's meant for seem to be serious car accidents where the seatbelt has become jammed or the occupant is trapped, so we're not addressing minor shunts and the forces involved before the hammer is required will be significant.
I'd ask a few questions to determine risks and benefits...
Can they be trusted not to touch it? If you press it to your head or other body parts (as kids often do) the autopunch part of this device will definitely hurt them. So if they're young enough for you to ask the question at all then they are too young.
Can they actually use it in an emergency? As much as it is purported to be easy to use by the manufacturers, in the confusion following an accident will they actually be able to use it to escape the vehicle? Do you have some spare seats around that you can use to train them? Even if you try will they even be strong enough to use it or have the dexterity to use it in that situation? If there is a question mark there again then the answer would be that they are still too young.
What are they going to do when they get out? As distressing as it may be to think a child might be stuck; what is even a young teenager going to do while dazed or shocked following an accident? In that state even adults have problems acting rationally and are going to be better off in the car where emergency services can find them and give help. So unless they're very mature and can cope under extreme pressure again it's probably not a good idea to install one for them.
Based on the risks, by the time a child gets to the level of capability and maturity needed for this kind of product to protect them they will be almost adult. So maybe if you really are sold on having one of these then the better thing to do is install one in the front of the car by the centre console where adults can get to it (or an older child can direct another adult to it) and in all other circumstances the risks will outweigh any benefits.