Your daughter can give you temporary permission to care for her child without any major formalities.  However, if the child is going to be with you for an extended period of time, consider having an attorney draft something more formal for you that gives you the power to consent to medical treatment and access to records involving the child.  If the child has any special needs where medical attention is a serious issue, you may want to consider requesting joint conservatorship with your daughter-- because some places will not accept a mere notarized document or power of attorney.For the first couple of options discussed above-- short time with you or power of attorney-- you do not need permission of dad.  As long as mom has the right to determine the residence of the child and make all of the major decisions (or if there is no order in place), then mom can designate any appropriate caregivers for her child.  For the joint conservatorship option, then dad would need to be served with notice of the custody suit.  He doesn't necessarily have to agree, but he is entitled to notice so that he can assert his parental rights if he choses to do so.