D.L.
Foster parents deal with children coming from such a variety of backgrounds, trauma, and situations that some may want to call you mom and dad and some could be offended by that. I would give them the choice since it is their childhood and you are there to help them. If you don't allow them to call you mom or dad when they want too they could feel like you want to keep a boundry between you and them or that they are diferent, not a true family member. For the children who don't want to call you mom you should have another name that is respectful but not too formal.