If something did happen, a well-trained 12 year old can do an awfull lot to help the 2 year old.

Being a Scout leader I spend a lot of time with kids that age (both Boy Scouts and Girl Scouts) and they have an amazing ability to learn and problem solve. Just make sure you clarify they they can't practice cutting the "real" seat belt or pop out the real window.

Which reminds me. If you do training, make sure you don't use the LifeHammer's blade while practicing. You want to keep that as sharp as possible. Maybe reserve a LifeHammer for practice only - on a seat belt strap for a junk yard.