Good rule of thumb; if you wouldn't do it with your grandmother, don't do it with anyone under 18.
There are 11 year olds who could pass for 18. Not many, but they are out there (my niece, for one). The problem is, if those investigating don't agree there was a reasonable basis for believing this girl was really 17, even though she told you 17 online, you have problems. That doesn't get you out of trouble entirely since you did know she was under 18. While what you did may fall under the grandmother rule above and not have violated any laws, staying out of legal trouble assumes the police come to the same conclusion. You have a ticked off father who would presumably not be so if he was under the belief you kept your hands to yourself, and an 11 year old who lies. You also admitted to drug use/possession and sharing those things with a minor.
There are 11 year olds who could pass for 18. Not many, but they are out there (my niece, for one). The problem is, if those investigating don't agree there was a reasonable basis for believing this girl was really 17, even though she told you 17 online, you have problems. That doesn't get you out of trouble entirely since you did know she was under 18. While what you did may fall under the grandmother rule above and not have violated any laws, staying out of legal trouble assumes the police come to the same conclusion. You have a ticked off father who would presumably not be so if he was under the belief you kept your hands to yourself, and an 11 year old who lies. You also admitted to drug use/possession and sharing those things with a minor.