Each online account can only file one return, and each online account incurs separate fees. I doubt Turbotax will decide to give $400 of free services to you because you pay for one tax return. You are one of the types of users that the Desktop program is designed form. The desktop program is generally more powerful and more capable, the only thing you don't get with the desktop program is cloud storage of your tax return.
If your children's income is less than $36,000, they can use the IRS Freefile program, which prepares even complicated returns for free, as long as the taxpayer is below the income limit. However, Turbotax no longer participates in IRS FreeFile. Turbotax online is going to charge you a separate free at the Premier level for each return with a K-1. You are much better off with the desktop program.
