sojane,

A mixed diet is best. They will not get all the nutrients just based of cereal and oats. Throw veggie and fruit scraps in as well and take them out within a day or two to prevent MOLD which will kill your colony. I have no experience with baby cereal. Sorry. But the preservatives in baby cereal may eventually have a negative effect. (not sure though)
https://www.airexotics.com/dubia-roach-care-sheet.html