J.C.
I wouldnt say tacky...though a bit annoying. Yes, she should have informed you that there was a cover charge to get in, IF she knew. Is it possible she didnt realize until she got there, and by then had already invited everyone...therefore couldnt change the venue?
Also, do you know where she stands financially? Maybe she wanted to cover everyone (pumpkin patches are usually pretty cheap, she may have thought she could cover it all) but then found it was out of her price?
Give her the benefit of the doubt. Ask in a polite way if she knew of the cover, and maybe let her know that next year she might want to note on the invitations if there is a price to pay to get into the place of the party -