With a group of that size have you thought of an
apartment hotel. I would recommend Aptos los Jeronimos on Calle de Moreto in Madrid. It just a short hop to the Prado. This is a great place two years ago when we went to Spain with 13 family members. Each aprtment had two baths and two bedrooms a living room and a small kitchen. We loved it because you could cook if you wanted and a fridge is very nice with children. Also you can catch the
Madrid Vision buses(you can ride all day for one fare and get on and off as much as you want) you catch them right in front of the Prado. Not much is ever mentioned about these buses on this site because most of the members are young (I think) but we loved them both in Madrid and Barcelona. A good idea is to get on the bus (either the blue or red)and ride the entire route and then on the second round you will know where you want to get off.
sabalagirl