You can add the expenses at a later time however, you would need to add them before you file your tax return. Otherwise, if you file your tax return without adding the childcare expenses you would be unable to make any changes unless your return is rejected. You would then have to wait until after you have received your tax refund and then Amend your tax return to add the expenses.
You can go ahead and enter any information that you have and you can return at a later time to add the childcare expenses and file your tax return. TurboTax Online will automatically save your progress, when you return you can pick up where you left off. To sign out click Sign Out at the lower left side of the program.