If I owe back child support and I’m claiming my girlfriend and her two kids can I file an injured spouse form so they can still get their refund

No.  You are not married.  You have to file a Joint return with a spouse to file the injured spouse form.  Does your girlfriend have any income?  You can't claim her unless she had less than 4,300 total income and lived with you the WHOLE year.  Is she filing her own return?  If she isn't filing a return what refund do you think she gets?

What?   If you have a GF how are you an injured "spouse?"   If YOU are the one who owes child support your legally married spouse could claim injured spouse on a joint return.   If you and your GF are not  legally married then injured spouse is not part of the plan.  And you do not "claim" a spouse as a dependent.   

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