How can I input my daughter and son's net income for 2016 and non-refundable tax credits if I haven't completed their 2016 tax return?

Typically, it's advised to complete the [generally] simpler returns for your dependants before you complete yours, so you have the amounts you need.

If you can't, estimate their income and amounts to the best of your ability and be prepared that your return may be re-assessed by the CRA to adjust your return when they process your dependant's returns.