Can a 10 year old see a 15 rated film with an adult?

If you attempted to see the movie at a public location such as the cinema, or if you watched it at a friends house without your parents consent, you have broken the law.

Cinemas have the legal right to ban anybody from the premises if they try to gain access to a movie while underage.

While with your own parents is a different story. Although it's "not advised", some cinemas allow underage viewers as long as they are accompanied by a biologically-related adult. You will have to contact the cinema directly to ask what their polices are on the matter.