Sorry, but neither of the prior posts are correct. Generally, if you add kids or any non-spouse as a joint tenant without them paying anything, that asset remains in your gross estate. (IRC 2040). The full value of the property contained in your gross estate will get a step up in basis. (IRC 1014).Also not technically advice here, but I think any gift during life (i.e., adding them to the deed as owners) means they keep the same basis that you had. I think stepped up basis only works if it passes as a result of death (pretty sure there's a term for it, but can't think right now) - so if it passes through an estate or through beneficiary, or even through a life tenancy, I think - which wouldn't apply, I don't think, to DVC. So if property is in someone name's as long as their alive, then goes to someone else upon death, I think that gets stepped up, but not 100%. If it passes because they're named as a beneficiary (investment account, bank account, etc.), it gets stepped up. Definitely if through an estate in some manner.
If it's gifted during life, which I think would apply to adding as co-tenants or joint tenants - meaning putting everyone's names on at the same time - then they keep the same cost basis.
The above post is getting confused with carry over basis on completed gifts. (IRC 1015).