It's about what you tax, not what for.

But then the tax can exceed the actual value of the land, in which case the net value of the land becomes negative. This is troubling. Imagine for example that due to taxes increasing or your income decreasing you no longer have the means to pay for your land. But you can't sell it either because it's value is negative! So you have to pay someone to take it away, but you might not have enough money. Moreover, if the size of the tax is disconnected from the actual value of the land, your "moral" justification for the tax falls apart.

The children have potential agency. I didn't account for that in my original post but I consider it relevant.

OK, so you need to introduce new rules about interaction with "potential agents".

It is interesting precisely because it is not already covered by some other concept...

I don't object to the concept of "violating agency is bad", I'm objecting to equating it with "morality" since this use of terminology is confusing. On the other hand, names are not a matter of great importance.

However since I believe taxes can be raised morally I don't consider the evil that is current forms of taxation to be necessary.

Even if taxes can be raised consistently with your agency rule (assuming it receives a more precise formulation), it doesn't follow it is the correct way to raise taxes since there are other considerations that have to be taken into account, which might be stronger.