hi Sally,
had the same problem with DS. He would cried and screamed the house down if I so much as to turn him over (before putting him on his tummy). Ask the paed, GP, and CHN about this and was suggested the towel thingy under his chest. Didn't work. DS still refused. Then my family GP said that I should just try different things see which one works cause he has to get used to being on his tummy otherwise it will be too hard for him to practice his neck muscle (as his head gets bigger). She continued on saying that her son was like that too but she found out that he was ok to be on his tummy as long as his nappy was off.
So I made a routine of giving DS a wash (with cloth and warm water) every morning. I undressed him and took his nappy off. After I gave him a quick front wipe, I told DS that I needed to clean his back and turned him over. I quickly wiped his back (and waited for him to cry :lol). He was fine. However he started to cry once I finished wiping him. So I figured out that so long as he is in the nude and I pretends that I am trying to clean him, he's fine.
So from then I try to prolong his daily wipe happy. I wipe him off twice for every section, dry him, put some lotion on, etc, then turn him over and do the same to his back. It has been 1.5 months now and he is getting very good on his tummy. He rarely complain anymore and now we are adding 'pure' tummy time in the afternoon (after nap), and the evening (before bath).
Sorry for the very long reply, but keep trying and you just have to persevere I think. Don't be put off when she cries, let her cry for a minute or two before you turn her over cause even a few minutes help them strengthen their neck muscle. Good luck!!