My son's girlfriend moved in with us when she was 3 months

My son's girlfriend moved in with us when she was 3 months pregnant with my son's first child, a daughter was born in June 2012. Everything appeared to be going well until November 2014, the mother admitted that she didn't love my son and had been having an affair for two months. She left our home that night leaving their daughter with us. The mother is of no true fixed abode at this time. My son paid for two appointments with a local solicitor and everything has been amicable since then. Our house is a small two bedroom property. We are at the point where my granddaughter needs to now move into a bed from a cot. My son and I discussed putting her bed in my room, I am a single divorcee. My son's ex has objected to this suggestion, reason being given is that my granddaughter will see me as her mother should she share my room. I am recently retired, I am 65 in September and the house is mortgaged to me. It is our intention to move to a three bedroom property, but this won't be financially possible until next year. Are we breaking army law if my little granddaughter shares my room until we move into a bigger property? Thank you.