... can I take my step-children out of state for spring break... their father has to work so he won't be attending the vacation until the last 2 days... he has given me consent, I have a power of attorney and their mother knows the vacation will be out of state..?

If you have consent from the parents it shouldn't be a problem.

Yes, I often take the children of friends on vacation with us, or kids of other family members, I see little or no difference

It doesn't appear to be a problem.