Don't worry if their interest fades or gets upstaged by something else. They may have already gained enough from the topic. Don't assume that just because your daughter was interested in hockey three weeks ago she still is. She may come full circle and become interested in hockey again, or she may not. It doesn't matter.
People become interested and disinterested in things all the time.
No child should be forced to study anything. People don't learn what they're not interested in. Not all people are interested in the same things, and that's okay. Not all people have to know the same things. Variety is important.