Covid tax relief, can I apply the $500 child credit at a later time if my 2018 taxes show only 1 child? My 2019 taxes will show 2 children, but not filed in time.

Yes. 

"In essence, the stimulus check acts as an advance of your 2020 income tax refund. This means when you prepare your 2020 income tax return, there will be a line to include the section 6428 credit. The credit on your 2020 return is subtracted by any amount received as a stimulus check in 2020. If the amount you received as a stimulus check is less than the credit you are due, the difference will be included as part of your 2020 refund. If you have been overpaid by receiving the stimulus check, however, you will not be required to return any excess amount".

Reference:

https://www.dailylocal.com/news/coronavirus/what-does-the-stimulus-check-mean-for-your-taxes/article...