During the breast feeding time alternatives would be a porridge, or animal milk. I assume you could grind food with a pestle and mortar but the baby would most likely die of malnourishment without it
Humans have only been farming grain for about 10 thousand years, and there's not many other foods you can grind. Ancient humans ate a lot of meat, and babies after weaning ate meat, much of it pre-chewed by mum. Things like brain wouldn't need to be chewed