In the US I'd pack in like sardines and wouldn't think twice. In Europe, and especially in Spain, I wouldn't risk it. I'd either book two rooms or look for another property. I would ask for adjoining rooms IF they have them. If not, I'd ask for AT LEAST rooms next to or directly across from each other.

Have you stayed in a hotel in Spain before? Two adults and two children in a room advertised as two twins is going to be TIGHT just to walk to the bathroom, much less sleep.