May I legally get custody of my Granddaughter ?

May I legally get custody of my Granddaughter ?
My son is in the service stationed in another state, he has sent me a general power of attorney to represent him in any matters concerning his daughter(7). My grandaughter and I are very close. Her mothers boyfriend slaps her, calls her names,she does not get the rest, meals, care that she needs, house is pig pen…I can no longer deal with her situation, she has stated she would like to live with me…before I call a Child Protection Agency, should I consult a lawyer and lay all the facts on the line..afraid if I say much she will run with her.Will the power of Attorney help..