I wouldn’t say that it is “WRONG, WRONG” but that it is vastly imprudent. Baptists have a strong missionary drive and it is very likely that the children whom they consider to be “non-Christian” (as they likely will consider your Catholic stepdaughter to be) will be proselytized at this camp. At 13, it is doubtful that your stepdaughter has the religious formation to fend off such attacks on her faith. For that reason, I can only recommend that Catholic parents avoid sending their children to Fundamentalist Protestant vacation Bible schools, camps, and private schools.
As a side note, I also recommend that your husband be the one to explain to his daughter why the two of you have decided not to allow her to go to this camp. Certainly it would be nice if he do so in your presence as that would uphold for her your joint authority. But the main thing to avoid is for you to explain to her a joint decision between her father and you that she cannot go on a trip she believes to be “a lot of fun,” because otherwise you may find yourself considered by her to be her “wicked stepmother.”