Thank you for your comments. I was able to talk to her about it, I stuck to the basics, reminded her about privacy & appropriate places. She also now has my stepsister's contact information, she knows her pretty well & can contact her if there's anything she doesn't feel comfortable talking with me about. I'm also upgrading our room to a 2 bedroom suite on our trip so she can have her own space, it's a little bit more money but I think it's for the best. Thanks again, this was really difficult for me to do but was I was able to get through it thanks to your helpful comments on this delicate situation.
I agree with the other posters that she just needs to be reminded about privacy. It is very important not to make her feel embarrassed about this, so either calmly and quickly remind her about privacy or as the others said find a woman to do it for you. Good luck!
I agree that this sexual discussion should take place with an appropriate older woman.
It is actually rather normal for kids to masturbate. Not all kids, but some kids. My cousin had a daughter who did at about age 2 and 3. She finally just told her daughter that this activity was something for her to do only in the privacy of her own bedroom when she was alone. If you cannot bring yourself to have this conversation with her, ask a trusted female friend to do it.
I would not share a room with a 10-year-old of the opposite sex at Disneyland, sorry for the expense but seriously, claims of sexual abuse by you are a lot more expensive to address, were she to get caught in some way when you were near.