I missed the child tax credit for 1998 and 1999 can I refile for this?

No. Any refund that you may have been due for the years 1998 and 1999 is no longer available to you.

Time for Filing a Claim for Refund

Generally, you must file a claim for a credit or refund within 3 years from the date you filed your original return or 2 years from the date you paid the tax, whichever is later. If you do not file a claim within this period, you may no longer be entitled to a credit or a refund.