How to replicate Cerelac at home?
You probably can't. 

Cereals are highly processed food, and they use methods not available at home. Producers also don't release information on the exact methods used in a certain product. 

If you just want to feed your baby a grain-based pap, look for recipes. Parents have fed grain products to babies long before Nestle existed. Old books for housewives and parents will have recipes, but there are probably recipes on the Internet too. 

Such recipes may produce something very different in taste, texture and digestive quality from the industrially-produced pap, although they will be suitable for babies. If you want exactly Cerelac, you will have to buy it. Even if there was a way to know exactly what they do to the grains, you'd probably need industrial equipment to replicate it.