With TurboTax Online, you pay per return. We have a promotion where you can get a deal on the cost of your returns when you're preparing your return together at the same time as your spouse, but that wouldn't work for two children.
If your children have pretty straightforward returns, you might want to consider TurboTax Free for them. You can check it out here: https://turbotax.intuit.ca/personal-tax-software/free-online.jsp