I am filling head of household, can I claim dependent care tax credit?

Hi, 

This question is about  Dependent care tax credit VS Dependent care tax credit

Last year my son went to daycare and the cost was deducted from my ex's FSA dependent care account(I paid her out of pocket cash though). I am going to claim him as my dependent and I am filling head of household status this year.

First and foremost:

Can a legally separate couple still claim dependent credit?  I read some articles saying that only married filing jointly can claim this credit, please correct me, if I am wrong.

If yes,

Can I claim my son in my tax return to get the dependent credit back?

Or

Has it to be her to claim my son as a dependent to get the dependent credit back? since she is the cardholder of the FSA account who paid for the daycare cost.

Sorry, I am trying to figure out this myself, but I really don't understand this part. Thanks in advance!