I have never heard this before. But I would have to say that IMHO, it makes no sense what so ever. I mean, take something like Caron's 100% acrylic Simply Soft and compare it to the 100% wool Noro? Although the problem with the latter could of course be due to the twigs and other foliage it's infamous for. But think of Lopi compared to Lion Brand Fun Fur, and I just can't think why someone said that acrylic is harder on your hands.
But given that this project is for a baby, here would be my suggestion-- no wool! Babies can be allergic to it, the cleaning is often more complicated, and new parents need to be able to throw it in the washing machine on a regular basis-- sometimes daily, knowing how infants have no control over either end of the their digestive tracts:) . Actually, the aforementioned Simply Soft is an excellent bet, quite cheap, and with many lines within that name. It'll work for socks as well, and babies' feet don't sweat, so it would still be a good choice.