As you are not able to claim any type of credits for your new baby on your 2018 "Income Tax Return", TurboTax will not allow a dependant's birthdate of 2019.
But don't worry, your Canada Child Benefits (CCB) will be adjusted according to your income in July as usual. The filing of your tax returns only ensures that you keep receiving the benefits and they are calculated correctly.
Here's a little more info on the calculation of those benefits: https://bit.ly/2NMXeMK
