Kitten food is richer in proteins & fats 'cause kittens (and lactating mothers) need much more enegry since they are growing. So, it's healthy, nothing dangerous about eating kitten food (unless your older cats have some health problems, food allergy, diarrhoea etc), but there is a possibility that your older cats can get fatter (if they aren't active enough).
I have the same problem. Minnosh is afraid of water sprays, so whenever I see her trying to eat the kittens' food, I show her the spray bottle (I don't have to spray, cause the bottle is enough to make her go away). As I did a few minutes ago
She is now eating from her own half light + half regular food bowl
If you have the chance, you can put the kitten's bowl to some place where only she can reach maybe ??