I went to Beijing last X'mas with my daughter. She liked the trip very much and we will go again in coming CNY. Many activites suitable to kids like skiing, play at science museum, visit Great Wall, Palace, bird nest, water cube etc.
I stayed in Novotel hotel. They allow 16 years old children stay with parents in same room with free and have free breakfast. You can go to their website to check whether they allow 2 kids to stay free.