relationships shouldn't particularly specify dating, in my opinion; it should be more general in terms of 'person to person relationships', including child to child, boyfriend to girlfriend, parent to child, parent to grandparent, etc. It includes boyfriend-girlfriend (dating), when the question is about the relationship factor.
I think it's okay that dating is partially a subset of this. Dating questions can sometimes overlap with relationships, and sometimes not:
What is appropriate to give as a gift for a first year dating anniversary?
(while not a great question here) would be fine for dating but not really relationships, while relationships would also fit on
My son is having a hard time understanding how to act with his longtime platonic friend who he is now dating.
I would suggest as a tag description for dating something like:
Questions about (older) children's romantic relationships.
Keep it simple. If it ends up nearly always having relationships on it, I don't think that's a big deal.
For relationships I think how it currently stands is okay, but we could add:
This refers to relationships between a child or group of children and other children, their siblings, or their parents. This could also refer to a relationship between parents in regards to the rearing of a child. See also dating for romantic relationships.
Not suggesting relationships is specifically wrong, as often the relationship aspect can be partially independent of the fact that it's a romantic relationship, but suggesting it as another tag.