Junk food warnings-"not for young children"

I recently watched a woman open a packet of salt and vinegar Monster Munch and hand it to a child in a buggy who must have been 18 months to 2 years old, who was obviously used to this and started eating. Apart from the damage to her developing taste buds the amount of salt and fat she consumed must have been health threatening. When I buy toys some have a warniing "Not suitable for children under the age of 3". Why can't the same warning be put on junk foods and fizzy drinks? Children might then eat better as they grow up.