Is it worth it to amend an accepted 2020 return if I can claim a child as a dependent (my ex wife didn't claim them so it's my year to and I didn't realize this).

It would be worth amending since the child tax credit alone is $2,000. If the child doesn’t live with you then your ex must give you a signed form 8332 to allow you to claim the child. 

Claiming a dependent on your tax return can reduce how much you owe.

The following credits may apply when you claim a dependent:

• Child Tax Credit.
• Additional child tax credit (ACTC)
• Other dependent credit (ODC)
• Earned income tax credit (EITC)
• Child and Dependent Care Credit

When you claim a dependent, you might be eligible to file as Head of household. Taxpayers who are eligible to file as head of household often pay lower taxes.