When I feed rice dumplings to my 3 year old son, I put anko in the center to avoid getting his mouth and face sticky from anko. He can eat it easier this way, and he stopped complaining,"No more bean paste!" It works perfectly.

The key is making the bean paste balls and freezing them beforehand. You can use unfrozen bean paste, but it will be difficult to wrap in the dough, because the dough might not be stretchy enough. It's easier to wrap frozen ones and they keep well in the freezer, so you're killing two birds with one stone. The bean paste is easy to defrost, so it's convenient to use for other recipes too. Also, the frozen koshi-an balls are delicious eaten straight from the freezer! Recipe by Makoto317