The preposition à (to, at) and de (of, from) both contract with the masculine article le and the plural article les — they never contract with la or l'. À + le becomes au (je vais au cinéma, I'm going to the cinema), and à + les becomes aux (je parle aux enfants, I'm talking to the children).
The same happens with de: de + le becomes du (le livre du professeur, the teacher's book), and de + les becomes des (la maison des voisins, the neighbors' house). These contractions are mandatory, not optional — à le and de les are simply not correct French, even though they'd be perfectly understandable.