Hi Tima,

The verb parler requires a preposition to complete its meaning.

Parler à --  to speak to   or   Parler de -- to speak of (about)  Therefore there needs to be an indirect object pronoun used  --  in this case "leur".

In your example "Je parle aux enfants"  --->  I speak "to" (preposition) the children.

"The children" is the indirect object of the verb parler.

Pronom d'objet indirect

Hope this helps

Jim