Options vary depending on your temperament and goals.

You could tell them no thanks, you are busy. Perfectly reasonable, it can be done kindly, and setting a clear boundary can work wonders.

You could also try managing rather than helping them. By this I mean helping them with the cognitive tasks of figuring out what they want to do and planning that out. This would mean finding a tutorial or video of a build, getting the summary of items needed, and then telling them how to go get those things and organise the storage.

If you go spelunking with them then having them come along as a part of the team and helping with something like strip mining can be very good. If they aren’t interested they can go do something else, but this specific task is something they can do to help. For example, I would make two long lines on the same level connected at one end to create a U shape then have them mine from one side to the other with a gap of two blocks. That simple mining is tedious but much better with extra hands, and the excitement of finding something good is enough to keep them interested often enough.