I think the vast majority of 12 year olds would be over 40kg and so would need adult jackets. If there are going to be younger (lighter) children also then you may just need to bite the bullet and buy some smaller ones also.
If these are for some kind of activity group rather than family use then foam may be a good choice, they are very robust and stand up well to being chucked around, shoved in lockers etc. Crewsaver Spiral is available in a 'Junior' size (30-40kg)
Spiral 100N | Crewsaver