"My parents always help us."
Translation:Mis padres siempre nos ayudan.
That would be incorrect. There are three situations (that I can think of) when you can attach the pronoun to the end of the verb.
infinitives: Puedo hacerlo (I can do it)
Positive Commands only (not negative ones): Olvídalo (Forget it)
Gerands: Estoy estudiandolo ahora (I am studying it now)
Think about it this way: Although a collective noun, "familia" is still considered a singular noun because it is in its singular form. Meanwhile, "padres" is a plural noun because it is in plural form (i.e., pluralized by an s). As with English, the singularity or plurality of a Spanish noun depends on whether or not a plural suffix (e.g., -s, -es) is added to it. See the examples below.
"Esa familia siempre nos ayuda." (That family always helps us.)
"Esas familias siempre nos ayudan." (Those families always help us.)
"Nuestro equipo tiene muchos jugadores." (Our team has many players.)
"Nuestros equipos tienen muchos jugadores." (Our teams have many players.)
"El grupo necesita más miembros." (The group needs more members.)
"Los grupos necesitan más miembros." (The groups need more members.)
*Note: The subject in each example is a collective noun.
I hope this is helpful.
"parents" = «padres»
I don't think any consideration of the gender mixture enters into it - it is just the word used.
If I used "parents" in English, you would not know what gender they are but the default assumption is still female/male. I think the same would be true when «padres» is used in Spanish.
Maybe others might have some comments on this (I'm not a native Spanish speaker, as you probably can guess)