It's not clear to me that legalization of those drugs would have the same effect as doing something else with the intent to encourage use of them. However, in the case of money, it's a little clearer: the only difference between giving someone extra money when they have another child and replacing money they spend on an extra child is what you say about it.

Now, it's true that what you say about it can have a real effect on choices and outcomes, but we don't really know enough to reliably predict what that effect is in the general case, so for practical purposes, I think we're better off assuming that the effects are negligible.