Baby fish need tiny bits of food and the only thing small enough for Betta fry is green water, infusoria or egg yolk.
You can buy fry food from pet shops. There is a powder fry food for egg layers and a liquid fry food for egg layers. The powder food is usually ground up algae, and the liquid fry food is usually based on egg yolk.
The following links have examples of commercially prepared fry food. The first link has a powdered fry food, the 2nd & 3rd links have a liquid fry food by a company called Wardleys, with the second link being the older packaging and the last link has the new packaging.
https://www.sera.de/en/product/freshwater-aquarium/sera-micron/
https://www.cichlid-forum.com/reviews/view_product_details.php?id=282
http://www.tails.co.nz/fish/food/wardley-small-fry-food
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Newly hatched brineshrimp are the most nutritious food for baby fish. Other small foods include vinegar eels and microworms, however baby brineshrimp is smaller and more nutritious than these.
You can buy dry brineshrimp eggs online or at any petshop and hatch them in salt water.
When the fry are a month old you can cut up dry tubifex and other foods that you feed to the adult fish, and feed them to the babies.