How to tell a child?

Ok, here's the situation. I am together with a divorced man with 2 children (10 and 7). Their mother is a member of the family, one of the main reasons why they are divorced. She takes them there every weekend which we don't like at all. We want to explain to the eldest what our concerns are, without freaking her out. Any suggestions? She herself thinks that the ppl in the family are really nice and friendly and she says things like "Jesus is my best friend", "Jesus talks to me" and "I am Jesus' soldier".
Any help is appreciated :)

Rubbyne