TurboTax Online is priced at a per return cost. Once you've signed into your account, select the option to start a new return. You'll prepare the returns one at a time for each child. If the returns are quite simple, you can start out in our Free version. It's perfect for less-complex returns. If you'd like the extra guidance Standard brings, you can upgrade at any time using the Upgrade feature near the bottom left of the return.
CRA allows up to 20 returns to be filed per computer.
https://turbotax.community.intuit.ca/replies/3288128