At what age must a woman wear hijab in front of non-mahram males, and when does kissing or touching non-mahram male children become impermissible?

There is no need to veil oneself from a non-discerning child. A discerning child who has not yet reached puberty is like a mahram in terms of looking. A pubescent child, however, is like an adult, and veiling from them is obligatory. A woman kissing a child is permissible and even recommended out of compassion and mercy, provided the child is young and not an object of desire.