I don't see an issue with two 13 year olds being boyfriend/girlfriend. It's the start of a long learning process for both, and as long as their parents are doing their jobs, it should be just fine.
As for a relationship with a non-JW, unfortunately that is a point they are firm on, generally. Any kind of association with non-JWs is risky, in their eyes. They use Bible verses that can imply a risk (1 Corinthians 15:33, for example), but I don't know if there are any that specifically forbid it. They're okay with the day-to-day interactions that are unavoidable (people at your job, where you shop, etc), but anything beyond that is frowned upon as a threat to your spiritual health.