Create an account for your child without administrator rights.
There must be everything you need, nothing superfluous.
Your account, of course, has administrator rights and password. The child's account must be without a password.
Installing your programs, specify "just for yourself." So the child will not play with your financial records, etc.
And is there something really useful on the Internet for kids?
Of course, there is both useful and harmful. You will never, never, never protect children from harmful, prohibiting. You have only the way to protect them, capturing something interesting for them.
Allow children to enter the Internet and independently orient in it, expecting that nothing will happen to him, that he will visit only good sites extremely silly and irresponsible.
It's all the same, what to bring him to the street and let himself choose.