DD was prescribed it recently after a trip to emergency with severe hives - she'd had an allergic reaction to penicillin. The emergency doctor said it's absolutely fine for babies but it's not recommended for GPs to prescribe it as many parents in the past had abused it and used it to help their kids sleep, IYKWIM

Not all kids relax on phernergan though and DD was one of those. She wasn't super hypo but it certainly didn't relax her. She slept well when she first had it, after we left emergency, but only because she was so sick and naturally exhausted. She was on phernergan until the hives disappeared, about 4-5 days, and it didn't help her sleep or chill out at all.

I know of older mums who swore by it to help their kids get extra sleep in the 70's, 80's and 90's, when it was readily available. I've only got one friend who is my age who's doctor actually prescribed it to help her child sleep.

ETA: the phernergan we have is in liquid form and it's specifically for infants.