"My grandparents are a little short."
Translation:Mis abuelos son un poco bajos.
I have worked out why. Poco is an adverb and Spanish adverbs do not change according to gender or number.
I believe its because when the person or persons are the subject you do not use the personal "a". Use it for direct objects. You have to watch "gustar" though because the "...is pleasing to..." translation makes the person/persons direct objects and therefore the "a" gets used. Please correct me if I am wrong.