I didn't give it directly to my son until he was eating solids, before then I could just load myself up and he'd get enough to be okay (slept fine even with a cold, but if I slacked off, his nose would stuff up and he'd have trouble sleeping at night cause he couldn't breathe through his nose) but if he'd gotten something potentially serious like WC, I would've given it directly to him even before solids.
I had to experiment to see how much my son needed, it really was trial-and-error. I dose at least 3x/day, if they're sick and I'm trying to figure out how much they need, I'll do it more often til I get loose stools, then back off. With practice, it gets pretty easy. At the beginning, I had to write things down.
From a practical standpoint, I know it's safe for a baby because my son's 2y9m now and a lot healthier than he used to be. Old posts by Momtezuma Tuatara are always worth a read. But she doesn't discuss (that I remember) how toxic load affects vitC need. She used it quite effectively for illness though, and one of the keys (that I've also found to be true) is that to make an impact, you really do need to be near bowel tolerance. Giving an adult an extra 1 or 2 grams of vitC per day when they're sick isn't going to do anything, and given how much my son tends to need, 1-2 grams wouldn't have done much for him either.
Nutribiotics is a good brand. I buy either Bronson (item 50B) or Now sodium ascorbate, we go through large quantities (I'm doing biomed with the kids and myself to get the mercury out, and daily bowel tolerance vitC has been a significant part of that, but I used it for illnesses before I knew what was going on with our health).