2 ideas that worked when I was in camp.
1) Have a daily davening raffle for the girls who davened beautifully (raffles can be given out after davening to avoid disruption of kavana) and the winner gets a spiritual prize (otherwise it looks like ice cream is better than davening…) we got Rebbe pictures – so whoever is the Gadol in your group / a Sefer… Works very well for turning the peer pressure the other way.
2) for more long lasting change, can combine with a brief davening thought / story before davening every day.
Good luck!! What can be accomplished in the 24 hour environment of camp can be more powerful than the entire school year for many children!