When you use TurboTax Online, you pay "per return". If you use the Self-Employed version, you are charged $59.99 for your own return and can prepare your spouse's additional return for $19.99.
For your children, you need to start these returns using the "Standard" version in order for the $19.99 fee to apply. And unfortunately, you cannot downgrade a return if you have started in the Self-Employed version. You will have to start a new one using Standard (or even Free).