Can I disallow purchases on my Xbox 360 even when I have points available?

On my Xbox 360 I have my account sign on automatically so that others in the household can watch Netflix. But I've noticed that now my account has 0 points because my 3 year old has purchased random stuff accidentally.

Is there a way to put points on my Live account but do something like password protect only the purchases?

Xbox support has been terribly unhelpful and hasn't given me a straight answer, so I'm guessing there's no way.

According to this Xbox support page, you can use the parental controls to restrict purchases, but only for a child account.

I'm not at home and can't verify this right now, but you should be able to create a new profile, designate it as a child account, and set that profile to sign in automatically. You can switch to your main profile when you want to make a purchase or play a game.

Not an optimal solution, but it's all I can think of.