"She demands to see her son."
Translation:Elle exige de voir son fils.
Demander à (+ infinitive): When "demander" and the verb in infinitive have the same subject, we use "demander à + infinitive": elle demande à le voir.
Demander de (+ infinitive). When the subject is different, we use "demander de + infinitive": il me demande de lui répondre.
In French, articles and possessive adjectives must agree in both gender and number with the noun that they describe. All French nouns are either masculine (le pain) or feminine (la maison). Mon, ma and mes are the three versions of "my" that agree with masculine, feminine and plural nouns. So my bread is mon pain. My house is ma maison and my houses translates to mes maisons. For "tu" (you familiar) form, ton, ta and tes are the three words. So your bread (ton pain), your house (ta maison) and your houses (tes maisons).
You didn't specifically ask and about the third person and hopefully I won't make this more confusing. The third person is the part that confused me when I first started learning French. The three possessive adjectives are son, sa and ses. Like the First (mon, ma, mes) and second (ton, tu, tes), son, sa and ses agree with the nouns they modify. The confusing part for me was that sa pain can mean his bread or her bread and sa maison can translate to his house or her house and ses maisons can translate to either his houses or her houses. In French, the correct word is still determined by the nouns's gender and number and not by the gender who owns the bread or house like it is in English.