Hey there sharonleemc,
It sounds like you want to make sure your Mac is safe from malware after your child opened up an email attachment accidentally. Is that correct?
Just to confirm, are you experiencing any unusual behavior on the Mac? If not, there may not be any need to worry. Especially if there was no personal information provided. MacOS is a secure operating system. With that being said there are a few steps you can take to help prevent malware. This includes installing the most recent version of macOS available.
Protect your Mac from malware
How to update the software on your Mac
You can also report the email as well. Let us know if you have any other questions.
Recognize and avoid phishing messages, phony support calls, and other scams
Cheers!