If UBI is thought of using existing fiat currency it may seem too expensive, but if we think about it as simply a number of votes given to all in order to determine how to make the best use of scarce resources, then there is no reason why a crypto currency that is simply given out each day or week might not work. The thought experiment I like to do is to imagine that robots can produce everything that we need, including fixing other robots. At that point, no one really needs to work, and no one really needs to own the robots or the scarce resources that they use to create what we need. However, since resources are almost always scarce, we need those votes to let the robots know how to use those resources to produce goods that meet our needs. As long as prices reflect scarcity and demand, this approach should work. You can save your votes, or spend them. Once spent, they disappear (remember robots are doing everything for us in this experiment, so you don't need to trade with other humans).
You bring up a good point above about the number of offspring. Clearly, more offspring means more needs, which in my thought experiment would make scarce resources even more scarce, thereby driving up costs. So there must be some mechanism for discouraging having children just to gain more UBI. A cap on "votes" per family should do this and have a similar effect to negative tax rate limits.
My main point here is that whenever UBI comes up, people start asking, "How can we afford this?" However, I think that crypto shows us that we CAN afford it, because it doesn't have to come from typical fiat currency or be thought of in the same manner.
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That's an interesting take on currency. I own a bit of crypto and it is an interesting field to watch develop over these past few years.
Maye your idea of just redistributing wealth with a USA-Coin, Canada-Coin, Brit-Coin or a Euro-Coin may be feasible in the near future.