I agree with the above 2 pps and think any of these are nice ideas. I would lean towards stopping at offering to do a fun thing witht he child (and family if they want to accompany), and probably not feel a gift card offer is necessary, but whichever makes you feel better.
I will say that this happened recently to one of ds' friend's mom. She told some of us the wrong day (said it was Sunday instead of Saturday). She actually figured out her error when my ds did not show up as she knows I am not the type to RSVP and no show--so she called us and we were able to hustle over and make it...a few kids were not notified until the party was over. This mom was only momentarily upset and didn't really do anything. I do think the idea of doing something special is nice and makes sense, but don't beat yourself up too much...it happens and helping kids learn this and deal with disappointment is the kids' parents' responsibilty.
lucky single mom to 20 yr old dd and 17 yr old ds through 2 very different adoption routes