I'm planing to visit Jiuzhaigou Valley on 1st of January with my family,my kids have 4 and 11 years.

Well, it's still ok for you to tour around the scenic area in January and you may see the snow and icy views. It can be quite safe and the elevation won't make your kids uncomfortable. Don't worry! By the way, the best timing to that valley is around Sep. to Oct. for colorful leaves and limpid lakes.