Hola Inma

Question

por qué es Mis padres se demorarán y no Mis padres les demorará?

Accepted Answer

Hola Mark
The verb demorarse is used in a way where the subject of the action is the person, so:

Yo me demoro 10 minutos en llegar al gimnasio. I take 10 minutes to get to the gym.
Mi hermana se demora una hora en llegar al trabajo. My sister takes an hour to get to work.
Mis amigos se demoran 20 minutos en llegar al restaurante. My friends take 20 minutes to get to the restaurant.

Here, with these translations in English, you can see how it's the same:  I take..., She takes..., They take... 
It can be confusing, and I imagine this is what may have confused you, if you see this other type of translation where the subject is not the person but it's "It":
It takes me 10 minutes to...
It takes her one hour to...
It takes them 20 minutes to...
Here, you can see that the structure is different, and it may make you want to use the indirect object pronoun: les demorará..., but we don't use it that way in Spanish normally.
I hope this is where you doubt was.
Saludos
Inma