I think this important for children to have a good habit.单句改

this 改为 it 或者it's （都是完全正确的）.
如果用it,则这是一个简单句,it在句中作形式宾语；而如果this 改为 it's,则这是一个主从复合句,it's important for children to have a good habit 是宾语从句,it 在宾语从句中充当句子的形式主语.