I need to access my sons account on my computer

Hello DaveH4365, welcome to the Microsoft community my name is Luiz Cruz, I am a Windows 10 expert user and would be happy to help you;

Is your child's account local or is it a Microsoft account?

MS Account:
Microsoft accounts can be retrieved from account.microsoft.com.

Local account:
before proceeding, it is important to note that all EFS encrypted files in the user account, including personal certificates, local and network/web passwords, etc. are stored. Since they are encrypted and therefore linked to the user's password, they cannot be recovered. And that this process requires you to be logged in as an administrator account.

Open Control Panel and go to > User Accounts > User Accounts (click the title again) > Manage another user account > Select account > Change password > Enter new password and select change password.

You can now login with the secondary account by logging out or restarting the machine. Again, this only applies to local accounts, those listed with an email address in the "Manage another user account" menu are Microsoft accounts and the user will need to retrieve them through any of Microsoft's own websites.

I hope I helped, see you soon!