When I used to dump baby guppies into Bixby's tank (betta, deceased) he was pretty inept at catching some of them. Especially since there were a lot of floating plants around, as I'm sure you will have in your betta tank. A few fry are just too smart, and eventually Bixby didn't bother with them any more and ended up having a few "guppy pets" in various stages of growth.
I learned to put them in 1 by 1 (a pain) right in front of his nose so he could snap them up in that first second when they were released. I still do that with my current 2 bettas. But it is the only way I can get rid of the excess. Just last week on cleaning day, in the 10 gallon, there were 30 new fry and, because of all the water sprite taking over there, too, the adult guppies aren't 'taking care' of them as much as I'd like.