You could try to mail in an amended return however I doubt this will work ... and if it did don't expect a check until Thanksgiving or latter.    The other option is to file a 2020 return next year for the missing stimulus money. 

So they will still give my dependent the stimulus money even though it is 2020 tax return?

The 2019 stimulus check is an ADVANCE of the 2020 credit ... so when you file the 2020 return the credit will be RECONCILED ... if you didn't get enough you get more.   If you got too much you get to keep it ... there is no "claw back" provision.

So I don't really need to file form 1040X to amend that non-filer tax return right? This question is just in case the other stimulus check will be accepted and my 18 years old could get some money.

If you are going to change anything on an already filed and accepted return (the non filer tool filed a real return for you)  then you must amend it to make any corrections.  However do you really want to do that ?   Any dependent over the age of 16 is not eligible for stimulus credit anyway. 

Well I heard that under 24 will be eligible for the next stimulus check, or it is wrong news?